bswap.h: fix GCC requirements for bswap* builtins

[rilysh]

Summary

1. __builtin_bswap{32,64} were added in GCC 4.3, and __builtin_bswap16 was added in GCC 4.8, however, currently, the GCC requirements in bswap.h file has >= 10. This requirement of GCC version is false for bswap* but true for __has_builtin() (as it first was added in GCC 10.1). As bswap* builtins were added before GCC 10, the preprocessor check will always going to be true for bswap but will be false if GCC version is < 10 as __has_builtin() won't be present. Since the byteswap function, on x86-64, can boil down to a single bswap instruction, this optimization may left behind (unless GCC do some pattern matching). To avoid this, just use the compiler macros (for GCC: _GNUC_, clang: _GNUC_ or _clang_) and if the compiler is neither GCC or Clang, fall-back to native implementation.

2. Remove the useless casts (uint{16,32,64}_t) from the constants. These constants already has their own suffix, and casting to a different type will just get ignored as the return value already gets casts to it's appropriate type.

3. Previously, Clang couldn't able to use __builtin_bswap* (even if it was newer) as LLVM define GNUC macro to a specific constant (usually lower than GCC's (GNUC) and on my system it's 4) which is indeed < 10. The first comment also fixes this issue.

Link: https://gcc.gnu.org/onlinedocs/gcc-4.3.0/gcc/Other-Builtins.html
Link: https://gcc.gnu.org/onlinedocs/gcc-4.8.0/gcc/Other-Builtins.html
Link: https://libc-alpha.sourceware.narkive.com/PfaB4BGP/patch-byteswap-h-fix-gcc-ver-test-for-builtin-bswap32

Checklist

• Closes #xxx
• I have discussed this with core contributors already
• This pull request requires changes to the ROM set
  • I have opened a roms pull request - https://github.com/86Box/roms/pull/changeme/

References

Provide links to datasheets or other documentation that helped you implement this pull request.