Define LONG64 if __SIZEOF_LONG__ indicates 64-bit long

All modern compilers (GCC>=4.6, Clang>=3.0) define this macro, so we can use it to detect 64-bit longs without adding to the architecture list.

This change is needed to successfully run xeyes on a FreeBSD CHERI-RISC-V QEMU VM via SSH forwarding. See also xorg/proto/xorgproto!41 (merged)

Signed-off-by: Alex Richardson Alexander.Richardson@cl.cam.ac.uk