Remove any trailing directory separator from ld_canon_sysroot and adjust ld_canon_sysroot_len.

The problem was the ld expect libiberty/lrealpath() to always return a
path, but in some cases it returns a prefix. It seemed like too much of
an earthquake to propose a fix to lrealpath.

	* ldmain.c (main): Remove directory separator, if any, from the
	end of the canonicalized sysroot.