Support TLS variables on FreeBSD/amd64.

Support TLS variables on FreeBSD/amd64.

Use the fs_base register to fetch the address of a thread's tcb and
calculate the address of the DTV array.  This value is then passed to
fbsd_get_thread_local_address to compute the final variable address.

Note that fs_base is currently only available via the native target as
core dumps on FreeBSD do not store the value of fs_base.

gdb/ChangeLog:

	* amd64-fbsd-tdep.c (amd64fbsd_get_thread_local_address): New.
	(amd64fbsd_init_abi): Install gdbarch
	"fetch_tls_load_module_address" and "get_thread_local_address"
	methods.