testsuite, Darwin: Handle __weak predefine.

Darwin predefines '__weak' for c-family which clashes with the use
in the test-case. Undefine it for the test purposes.

gcc/testsuite/ChangeLog:

	* gcc.dg/lto/attr-weakref-2_0.c: Undefine '__weak' for this
	test.

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