Testsuite: Add gdb_can_simple_compile

Simplfy gdb.exp by adding a function that will attempt to
compile a piece of code, then clean up.

gdb/testsuite

	* lib/gdb.exp (gdb_can_simple_compile): Add proc.
	(support_complex_tests): Use gdb_can_simple_compile.
	(is_ilp32_target): Likewise.
	(is_lp64_target): Likewise.
	(is_64_target): Likewise.
	(is_amd64_regs_target): Likewise.
	(is_aarch32_target): Likewise.
	(gdb_int128_helper): Likewise.