compiler: Add -fgo-pkgpath option.

compiler: Add -fgo-pkgpath option.

	* lang.opt: Add -fgo-pkgpath.
	* go-lang.c (go_pkgpath): New static variable.
	(go_prefix): New static variable.
	(go_langhook_init): Pass go_pkgpath and go_prefix to
	go_create_gogo.
	(go_langhook_handle_option): Handle -fgo-pkgpath.  Change
	-fgo-prefix handling to just set go_prefix.
	* go-c.h (go_set_prefix): Don't declare.
	(go_create_gogo): Add pkgpath and prefix to declaration.
	* go-gcc.cc (Gcc_backend::global_variable): Change unique_prefix
	to pkgpath.  Don't include the package name in the asm name.
	* gccgo.texi (Invoking gccgo): Document -fgo-pkgpath.  Update the
	docs for -fgo-prefix.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@187356 138bc75d-0d04-0410-961f-82ee72b054a4