libstdc++: Refactor dejagnu effective-target checks

libstdc++: Refactor dejagnu effective-target checks

This introduces two new procs to replace boilerplate in the
effective-target checks.

libstdc++-v3/ChangeLog:

	* testsuite/lib/libstdc++.exp (v3_try_preprocess): Define
	new proc to preprocess a chunk of code.
	(v3_check_preprocessor_condition): Define new proc to test
	a preprocessor condition depending on GCC or libstdc++ macros.
	(check_v3_target_debug_mode, check_v3_target_normal_mode):
	Use v3_try_preprocess.
	(check_v3_target_normal_namespace)
	(check_v3_target_parallel_mode, check_v3_target_cstdint)
	(check_v3_target_cmath, check_v3_target_atomic_builtins)
	(check_v3_target_gthreads, check_v3_target_gthreads_timed)
	(check_v3_target_sleep, check_v3_target_sched_yield)
	(check_v3_target_string_conversions, check_v3_target_swprintf)
	(check_v3_target_binary_io, check_v3_target_nprocs): Use
	v3_check_preprocessor_condition.
	(check_effective_target_cxx11): Likewise.
	(check_effective_target_random_device): Likewise.
	(check_effective_target_tbb-backend): Likewise.
	(check_effective_target_futex): Likewise.
	(check_v3_target_little_endian) Call check_effective_target_le.
	(check_effective_target_atomic-builtins): New proc to define
	new effective-target keyword.
	(check_effective_target_gthreads-timed): Likewise.