Darwin: homogenize spelling of macOS

gcc/ChangeLog:
	* config.in: Regenerate.
	* config/darwin-c.cc: Change spelling to macOS.
	* config/darwin-driver.cc: Likewise.
	* config/darwin.h: Likewise.
	* configure.ac: Likewise.
	* doc/contrib.texi: Likewise.
	* doc/extend.texi: Likewise.
	* doc/invoke.texi: Likewise.
	* doc/plugins.texi: Likewise.
	* doc/tm.texi: Regenerate.
	* doc/tm.texi.in: Change spelling to macOS.
	* plugin.cc: Likewise.

gcc/analyzer/ChangeLog:
	* kf.cc: Change spelling to macOS.

gcc/c-family/ChangeLog:
	* c.opt: Change spelling to macOS.

gcc/fortran/ChangeLog:
	* gfortran.texi: Likewise.

gcc/jit/ChangeLog:
	* jit-playback.cc: Change spelling to macOS.

gcc/objc/ChangeLog:
	* objc-act.cc: Change spelling to macOS.