Disable preprocessing of assembler files with m4 on non-MIPS systems

Disable preprocessing of assembler files with m4 on non-MIPS systems

* exec/Makefile.in (is_mips): New substitution.
(.s.o): Avoid calling m4 unless it be set.

* exec/configure.ac: Substitute is_mips, and don't generate
config-mips.m4 otherwise.

* exec/loader-x86.s:

* exec/loader-x86_64.s: Use assembler rather than m4-defined
comment syntax.

(cherry picked from commit fa322155ad51e3628256232bc113ad7499ab76e7)