Some of the solution to PR71767 is incomplete, and we need finer-grained
control over whether symbols need to be made linker-visible.  This is a
preparation patch, providing the flag.

gcc/ChangeLog:

2019-11-10  Iain Sandoe  <iain@sandoe.co.uk>

	* config/darwin.h (MACHO_SYMBOL_FLAG_LINKER_VIS): New.
	(MACHO_SYMBOL_LINKER_VIS_P): New.

From-SVN: r278028