Add C99-style constructor macros for fixed types to
<asm-generic/int-*.h>.  Since Linux uses names like "u64" instead of
"uint64_t", the constructor macros are called U64_C() instead of
UINT64_C() and so forth.

These macros allow specific sizes to be specified as
U64_C(0x123456789abcdef), without gcc issuing warnings as it will if
one writes (u64)0x123456789abcdef.

When used from assembly, these macros pass their argument unchanged.
Signed-off-by: NH. Peter Anvin <hpa@zytor.com>