common: Move {BITS,BYTES}_PER_LONG into limits.h

common: Move {BITS,BYTES}_PER_LONG into limits.h

We're already using compiler-provided __*_TYPE__ macros, so instead of basing
BYTES_PER_LONG on __i386__ or __x86_64__, just use __SIZEOF_LONG__ directly.

This form doesn't require editing to add different architectures.

Add some build assertions that we have a half-way sane compile environment.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>