stdint.h: Make {u}int64_t types consistent regardless of arch

On 64-bit arch's like AARCH64, x86_64, or riscv-64 the way {u}int64_t
will differ from how its defined on 32-bit arch's.  Do the same trick we
do for {u}int32_t for {u}int64_t so that the type is the same regardless
of what we are building for.

This keeps things like %lld working the same regardless of 32-bit or
64-bit arch.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>