Prefer uint64_t for the definition of u64.

* src/types.h (u64): Prefer use of uint64_t with C99 compliant
compiler.

In libgcrypt, we use u64 for 64-bit unsigned integer, it's before C99.

Newer jitterentropy introduces use of uint64_t.

In LP64 systems, there are two variants: (1) uint64_t is defined as
unsigned long, (2) uint64_t is defined as unsigned long long.

macOS is the latter.

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>