Treat a prefix of "$SYSROOT" in the same way as "=" when parsing linker search paths.

Treat a prefix of "$SYSROOT" in the same way as "=" when parsing linker search paths.

	PR ld/21251
	* ldfile.c (ldfile_add_library_path): If the path starts with
	$SYSROOT then use the sysroot as the real prefix.
	* ldlang.c (lang_add_input_file): Treat $SYSROOT in the same
	way as =.
	* ldlex.l: Add $SYSROOT as allow prefix for a filename.
	* ld.texinfo (-L): Document that $SYSROOT acts like = when
	prefixing a library search path.
	(INPUT): Likewise.
	* testsuite/ld-scripts/sysroot-prefix.exp: Add $SYSROOT prefix
	tests.