The big endian RISC-V effort is mostly driven by MIPS (the company) which is
pivoting to RISC-V, and presumably needs a big endian variant to fill the niche
that big endian MIPS (the ISA) did.
GCC already supports these targets, but LLVM support will only appear in 22;
this commit just adds the necessary target knowledge and checks on our end.
This code applies to ~any POSIX OS where we don't link libc. For example, it'll
be useful for FreeBSD and NetBSD.
As part of this, move std.os.linux.pie to std.pie since there's really nothing
Linux-specific about what that file is doing.
