compiler.h: don't define inb/outb and friends on mips

The definition relies on IOPortBase, which is only ever set in
hw/xfree86/os-support/bsd/arm_video.c

This caused build failures on linux/mips with GCC 10, due to this
change (from https://gcc.gnu.org/gcc-10/changes.html#c):

"GCC now defaults to -fno-common. As a result, global variable accesses
are more efficient on various targets. In C, global variables with
multiple tentative definitions now result in linker errors. With
-fcommon such definitions are silently merged during linking."

As a result anything including compiler.h would get its own definition
of IOPortBase and the linker would error out.
4 jobs for mips-gcc10-build-fix in 12 minutes and 26 seconds (queued for 1 second)