std.debug: FP-based unwinding is impossible on avr, csky, msp430, and xcore

The ABIs do not define a frame pointer register, nor do they define a guaranteed
and fixed area on the stack where one might find saved registers such as a frame
pointer or return address.

lib/std/debug.zig

@@ -872,10 +872,14 @@ const StackIterator = union(enum) {
     };
 
     const fp_usability: FpUsability = switch (builtin.target.cpu.arch) {
+        .avr,
+        .csky,
         .mips,
         .mipsel,
         .mips64,
         .mips64el,
+        .msp430,
+        .xcore,
         => .useless,
         .hexagon,
         // The PowerPC ABIs don't actually strictly require a backchain pointer; they allow omitting