testsuite: fix is_amd64_regs_target

Commit

    c221b2f7 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_amd64_regs_target
checks to fail.  In gdb.btrace/tailcall.exp and others, this causes the
wrong source file to be picked and the test to fail on 64-bit targets.

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

testsuite/
	* lib/gdb.exp (is_amd64_regs_target): Change assembly to C inline
	assembly.