target/xtensa: Clean up core-isa.h header guards

target/xtensa: Clean up core-isa.h header guards

scripts/clean-header-guards.pl warns these headers use reserved
identifier _XTENSA_CORE_CONFIGURATION_H as header guard symbol.  It
additionally warns the guard doesn't match the file name.

Reuse of the same guard symbol in multiple headers is okay as long as
they cannot be included together.

Since we can avoid guard symbol reuse easily, do so: use the guard
symbol scripts/clean-header-guards.pl picks, less the TARGET_ prefix.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20190315145123.28030-5-armbru@redhat.com>