build: Include cpuopts.h in "config.h"

The Autoconf/Automake build system did something similar.  This fixes
the build of sigprogmask.c which uses RTEMS_POSIX_API and expects to get
it defined via "config.h".

Update #3818.

spec/build/cpukit/cfghdr.yml

@@ -4,7 +4,8 @@ copyrights:
 - Copyright (C) 2020 embedded brains GmbH (http://www.embedded-brains.de)
 enabled-by: true
 guard: ''
-include-headers: []
+include-headers:
+- rtems/score/cpuopts.h
 install-path: null
 links:
 - role: build-dependency