Add a new gdbarch method to resolve the address of TLS variables.

Add a new gdbarch method to resolve the address of TLS variables.

Permit TLS variable addresses to be resolved purely by an ABI rather
than requiring a target method.  This doesn't try the target method if
the ABI function is present (even if the ABI function fails) to
simplify error handling.

gdb/ChangeLog:

	* gdbarch.sh (get_thread_local_address): New method.
	* gdbarch.h, gdbarch.c: Regenerate.
	* target.c (target_translate_tls_address): Use
	gdbarch_get_thread_local_address if present instead of
	target::get_thread_local_address.