std.debug: greatly expand target support for segfault handling/unwinding

I made a couple of decisions for this based on the fact that we don't expose the
signal_ucontext_t type outside of the file:

* Adding all the floating point and vector state to every ucontext_t and
  mcontext_t variant was way, way too much work, especially when we don't even
  use the stuff. So I deleted all that and kept only the bare minimum needed to
  reach into general-purpose registers.
* There is no particularly compelling reason to stick to the naming and struct
  nesting used in the system headers. So we can actually unify the access
  patterns for almost all of these variants by taking some liberties here; as a
  result, fromPosixSignalContext() is now much nicer to read and extend.

lib/std/debug.zig

@@ -1315,15 +1315,26 @@ fn getDebugInfoAllocator() Allocator {
 
 /// Whether or not the current target can print useful debug information when a segfault occurs.
 pub const have_segfault_handling_support = switch (native_os) {
+    .haiku,
     .linux,
-    .macos,
-    .netbsd,
-    .solaris,
-    .illumos,
-    .windows,
-    .freebsd,
-    .openbsd,
     .serenity,
+
+    .dragonfly,
+    .freebsd,
+    .netbsd,
+    .openbsd,
+
+    .driverkit,
+    .ios,
+    .macos,
+    .tvos,
+    .visionos,
+    .watchos,
+
+    .illumos,
+    .solaris,
+
+    .windows,
     => true,
 
     else => false,
@@ -1406,11 +1417,26 @@ fn handleSegfaultPosix(sig: i32, info: *const posix.siginfo_t, ctx_ptr: ?*anyopa
             }
         }
         const addr: usize = switch (native_os) {
-            .linux => @intFromPtr(info.fields.sigfault.addr),
-            .freebsd, .macos, .serenity => @intFromPtr(info.addr),
-            .netbsd => @intFromPtr(info.info.reason.fault.addr),
-            .openbsd => @intFromPtr(info.data.fault.addr),
-            .solaris, .illumos => @intFromPtr(info.reason.fault.addr),
+            .serenity,
+            .dragonfly,
+            .freebsd,
+            .driverkit,
+            .ios,
+            .macos,
+            .tvos,
+            .visionos,
+            .watchos,
+            => @intFromPtr(info.addr),
+            .linux,
+            => @intFromPtr(info.fields.sigfault.addr),
+            .netbsd,
+            => @intFromPtr(info.info.reason.fault.addr),
+            .haiku,
+            .openbsd,
+            => @intFromPtr(info.data.fault.addr),
+            .illumos,
+            .solaris,
+            => @intFromPtr(info.reason.fault.addr),
             else => comptime unreachable,
         };
         const name = switch (sig) {

lib/std/debug/SelfInfo/Elf.zig

@@ -94,6 +94,15 @@ pub const can_unwind: bool = s: {
     // Notably, we are yet to support unwinding on ARM. There, unwinding is not done through
     // `.eh_frame`, but instead with the `.ARM.exidx` section, which has a different format.
     const archs: []const std.Target.Cpu.Arch = switch (builtin.target.os.tag) {
+        // Not supported yet: arm, m68k, sparc64
+        .haiku => &.{
+            .aarch64,
+            .powerpc,
+            .riscv64,
+            .x86,
+            .x86_64,
+        },
+        // Not supported yet: arc, arm/armeb/thumb/thumbeb, csky, m68k, or1k, sparc/sparc64, xtensa
         .linux => &.{
             .aarch64,
             .aarch64_be,
@@ -113,31 +122,54 @@ pub const can_unwind: bool = s: {
             .x86,
             .x86_64,
         },
+        .serenity => &.{
+            .aarch64,
+            .x86_64,
+            .riscv64,
+        },
+
+        .dragonfly => &.{
+            .x86_64,
+        },
+        // Not supported yet: arm
+        .freebsd => &.{
+            .aarch64,
+            .powerpc64,
+            .powerpc64le,
+            .riscv64,
+            .x86_64,
+        },
+        // Not supported yet: arm/armeb, m68k, mips64/mips64el, sparc/sparc64
         .netbsd => &.{
             .aarch64,
             .aarch64_be,
+            .mips,
+            .mipsel,
+            .powerpc,
             .x86,
             .x86_64,
         },
-        .freebsd => &.{
-            .x86_64,
-            .aarch64,
-        },
+        // Not supported yet: arm, sparc64
         .openbsd => &.{
+            .aarch64,
+            .mips64,
+            .mips64el,
+            .powerpc,
+            .powerpc64,
+            .riscv64,
+            .x86,
             .x86_64,
         },
-        .solaris => &.{
-            .x86_64,
-        },
+
         .illumos => &.{
             .x86,
             .x86_64,
         },
-        .serenity => &.{
+        // Not supported yet: sparc64
+        .solaris => &.{
             .x86_64,
-            .aarch64,
-            .riscv64,
         },
+
         else => unreachable,
     };
     for (archs) |a| {