Fix infinite recursion in amd64fbsd_sigcontext_addr

Fix infinite recursion in amd64fbsd_sigcontext_addr

amd64fbsd_sigcontext_addr is using frame_unwind_register_unsigned to
fetch the stack pointer which results in infinite recursion.  This
patch changes it to use get_frame_register to match the
sigcontext_addr methods in the i386-bsd and amd64-linux targets
instead.

gdb/ChangeLog:
2015-02-25  John Baldwin  <jhb@freebsd.org>

	* amd64fbsd-tdep.c (amd64fbsd_sigcontext_addr): Use
	get_frame_register instead of frame_unwind_register_unsigned.