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 ...

LICENSES/exceptions/GCC-exception-2.0

+SPDX-Exception-Identifier: GCC-exception-2.0
+SPDX-URL: https://spdx.org/licenses/GCC-exception-2.0.html
+SPDX-Licenses: GPL-2.0, GPL-2.0+, GPL-2.0-only, GPL-2.0-or-later
+Usage-Guide:
+  This exception is used together with one of the above SPDX-Licenses to
+  allow linking the compiled version of code to non GPL compliant code.
+  To use this exception add it with the keyword WITH to one of the
+  identifiers in the SPDX-Licenses tag:
+    SPDX-License-Identifier: <SPDX-License> WITH GCC-exception-2.0
+License-Text:
+
+In addition to the permissions in the GNU Library General Public License,
+the Free Software Foundation gives you unlimited permission to link the
+compiled version of this file into combinations with other programs, and to
+distribute those programs without any restriction coming from the use of
+this file. (The General Public License restrictions do apply in other
+respects; for example, they cover modification of the file, and
+distribution when not linked into another program.)