Fix building gdb with gcc-4.x

Fix building gdb with gcc-4.x

Since is_trivially_default_constructible was not implemented before gcc-5
it cannot be used with gcc-4.x.

Fix the build by using conditional compilation around that line.
Use the equivalent is_trivially_constructible<T> instead, since
we already have HAVE_IS_TRIVIALLY_CONSTRUCTIBLE for that purpose.

Fixes: 098caef485a ("Refactor struct trad_frame_saved_regs")
gdb:
2021-01-14  Bernd Edlinger  <bernd.edlinger@hotmail.de>

	* trad-frame.c (trad_frame_alloc_saved_regs): Avoid compile-error
	because is_trivially_default_constructible was first implemented with
	gcc-5.