Support the fs_base and gs_base registers on FreeBSD/amd64 native processes.

Support the fs_base and gs_base registers on FreeBSD/amd64 native processes.

Use ptrace operations to fetch and store the fs_base and gs_base registers
for FreeBSD/amd64 processes.  Note that FreeBSD does not currently store the
value of these registers in core dumps, so these registers are only
available when inspecting a running process.

gdb/ChangeLog:

	* amd64-bsd-nat.c (amd64bsd_fetch_inferior_registers): Use
	PT_GETFSBASE and PT_GETGSBASE.
	(amd64bsd_store_inferior_registers): Use PT_SETFSBASE and
	PT_SETGSBASE.