compiler.h: only use inx/outx on ARM with glibc

musl only implements inx/outx on x86, so check for __GLIBC__ instead of
__linux__.

Signed-off-by: Ross Burton <ross.burton@intel.com>
1 job for musl in 2 minutes and 11 seconds (queued for 3 seconds)
Status Name Job ID Coverage
  Test
passed job #25173

00:02:11