Add x86_64 ravenscar support

Add x86_64 ravenscar support

Support for x86_64 ravenscar was recently added to the Ada runtime.
This patch updates gdb to follow.

As this is Ada-specific, and was reviewed internally by Joel, I am
checking it in.

2020-11-02  Tom Tromey  <tromey@adacore.com>

	* Makefile.in (ALL_64_TARGET_OBS): Add amd64-ravenscar-thread.o.
	(ALLDEPFILES): Add amd64-ravenscar-thread.c.
	(HFILES_NO_SRCDIR): Add amd64-ravenscar-thread.h.
	* amd64-ravenscar-thread.c: New file.
	* amd64-ravenscar-thread.h: New file.
	* amd64-tdep.c (amd64_init_abi): Register ravenscar ops.
	* configure.tgt (amd64_tobjs): Add ravenscar objects.