Darwin: Make sanitizer local vars linker-visible.

Darwin: Make sanitizer local vars linker-visible.

Another case where we need a linker-visible symbols in order to
preserve the ld64 atom model.  If these symbols are emitted as
'local' the linker cannot see that they are separate from any
global weak entry that precedes them.  This will cause the linker
to complain that there is (apparently) direct access to such a
weak global, preventing it from being replaced.

This is a short-term fix for the problem - we need generic
handling for relevant cases (that also does not pessimise objects
by emitting unnecessary symbols and relocations).

gcc/ChangeLog:

2020-05-23  Iain Sandoe  <iain@sandoe.co.uk>

	* config/darwin.h (ASM_GENERATE_INTERNAL_LABEL):
	Make ubsan_{data,type},ASAN symbols linker-visible.