libgcc, emutls: Allow building weak definitions of the emutls functions.

libgcc, emutls: Allow building weak definitions of the emutls functions.

In order to better support use of the emulated TLS between objects with
DSO dependencies and static-linked libgcc, allow a target to make weak
definitions.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>
libgcc/ChangeLog:

	* config/t-darwin: Build weak-defined emutls objects.
	* emutls.c (__emutls_get_address): Add optional attributes.
	(__emutls_register_common): Likewise.
	(EMUTLS_ATTR): New.