Fix build on gcc < 5 (std::is_trivially_copyable missing)

Fix build on gcc < 5 (std::is_trivially_copyable missing)

Ref: https://sourceware.org/ml/gdb-patches/2017-04/msg00660.html

Simply skip the poisoning on older compilers.

gdb/ChangeLog:
2017-04-25  Pedro Alves  <palves@redhat.com>

	* common/poison.h [!HAVE_IS_TRIVIALLY_COPYABLE] (IsRelocatable)
	(BothAreRelocatable, memcopy, memmove): Don't define.
	* common/traits.h (__has_feature, HAVE_IS_TRIVIALLY_COPYABLE): New
	macros.