testsuite: fix is_aarch32_target

testsuite: fix is_aarch32_target

Commit

   c221b2f77080 Testsuite: Add gdb_can_simple_compile

changed the source file name extension of the test program from .s to .c
resulting in compile fails.  This, in turn, causes is_aarch32_target
checks to fail.

Change the test source from an assembly program to a C program using
inline assembly.

is_amd64_regs_target had a similar problem, which was fixed by commit

    224d30d39365 testsuite: fix is_amd64_regs_target

This fix — and commit message — are mostly copied from it.

Approved-By: Simon Marchi <simon.marchi@efficios.com>