LICENSES: Add GCC runtime library exception text

A recent commit added SPDX identifiers to the SuperH low level library code
which originates from GCC. This code is licensed under the GPL 2.0 or later
with the GCC runtime library exception.

Unfortunately the authors did not bother to add the exception text to the
LICENSES directory so spdxcheck fails with:

 arch/sh/lib/ashiftrt.S: 1:42 Invalid Exception ID: GCC-exception-2.0
 arch/sh/lib/ashlsi3.S: 1:42 Invalid Exception ID: GCC-exception-2.0
 arch/sh/lib/ashrsi3.S: 1:42 Invalid Exception ID: GCC-exception-2.0
 arch/sh/lib/lshrsi3.S: 1:42 Invalid Exception ID: GCC-exception-2.0
 arch/sh/lib/movmem.S: 1:42 Invalid Exception ID: GCC-exception-2.0
 arch/sh/lib/udiv_qrnnd.S: 1:42 Invalid Exception ID: GCC-exception-2.0
 arch/sh/lib/udivsi3.S: 1:42 Invalid Exception ID: GCC-exception-2.0
 arch/sh/lib/udivsi3_i4i-Os.S: 1:42 Invalid Exception ID: GCC-exception-2.0
 arch/sh/lib/udivsi3_i4i.S: 1:42 Invalid Exception ID: GCC-exception-2.0

Add the exception text ...