gen_matypes: fix cross-compiling with gcc

gen_matypes: fix cross-compiling with gcc

The current gen_matypes logic assumes that the host compiler will produce
information that is useful for the target compiler.  Unfortunately, this
is not the case whenever cross-compiling.

When we detect that we're cross-compiling and using GCC, use the target
compiler to produce assembly from the gen_matypes.c source, then process
it with a shell script to create a usable header.  This is similar to how
the linux kernel creates its asm-offsets.c file.

Reviewed-by: Matt Turner <mattst88@gmail.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>