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 allows me to successfully run a simple X11 window on a
64-bit FreeBSD RISC-V QEMU VM via SSH forwarding. Without this change
I get an error that DISPLAY cannot be opened.

Signed-off-by: Alex Richardson <Alexander.Richardson@cl.cam.ac.uk>
10 jobs for fix-riscv64 in 5 minutes and 21 seconds (queued for 1 second)
Status Name Job ID Coverage
  Prep
passed container-prep #10885985

00:03:56

 
  Check
passed check formatting #10885986

00:00:14

 
  Build
passed autotools: [] #10885989

00:00:42

passed autotools: [--enable-legacy] #10885990

00:00:42

passed meson: [] #10885987

00:00:13

passed meson: [-Dlegacy=true] #10885988

00:00:13

 
  Test
passed check evdev keysyms #10885993

00:00:14

passed check versions are in sync #10885994

00:00:19

passed compare meson and autotools #10885992

00:00:26

passed meson from tarball #10885991

00:00:15