Clean up x86 endian.h:

Clean up x86 endian.h:
- Remove extern "C". There are no functions with external linkage here. [1]
- Rename bswapNN_const(x) to bswapNN_gen(x) to indicate that these macros
  are generic implementations that can take non-constant arguments. [1]
- Split up __GNUCLIKE_ASM && __GNUCLIKE_BUILTIN_CONSTANT_P and deal with
  each separately.
- Replace _LP64 with __amd64__ because asm instructions are machine
  dependent, not ABI dependent.

Submitted by:	bde [1]
Reviewed by:	bde