Darwin, objective-c - register gnu-runtime headers correctly.

Darwin, objective-c - register gnu-runtime headers correctly.

Darwin is able to use two runtimes for objective-c; the
default is its native "NeXT" runtime, but also it can build
code using the "gnu-runtime".  In order to do this, we have to
be able to find the gnu-runtime headers (which are installed
into the compiler's tree).

The process to do this is erroneously prepending the sysroot
to this when a sysroot is in force.  The gnu-runtime headers have
never been installed in a Darwin (macOS) SDK so we must make
sure that they are found local to the compiler.

gcc/

2019-05-18  Iain Sandoe  <iain@sandoe.co.uk>

	* config/darwin-c.c (darwin_register_objc_includes): Do not
	prepend the sysroot when building gnu-runtime header search
	paths.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@271371 138bc75d-0d04-0410-961f-82ee72b054a4