Fix an incorrect sed regular expression used in libjava by adding a local
patch files/patch-libjava-classpath-scripts-check_jni_methods.sh.in.
Escapes of ordinary characters will be disallowed by regex(3) in the
future. [1]

While here already, factor out a proper USES block in the Makefile. [2]

PR: 233443 [1]
Submitted by: kevans [1]
Reported by: portlint [2]