If "=" is the first character in a ldscript input file, force a sysroot prefix.

If "=" is the first character in a ldscript input file, force a sysroot prefix.

	* ldlang.c (lang_add_input_file): If the first character in the
	filename is '=', prepend the sysroot and force the context of that
	input file to non-sysroot.

The "input_flags.sysrooted = 0" thing described in the comment is
covered by the testsuite part ("root-anchored =-prefixed script
inside"), but only observable for --with-sysroot configurations.