[S390] __builtin_trap() and gcc version.

__builtin_trap() has the archictecture defined backend in gcc since gcc 3.3.
To make sure the kernel builds with gcc 3.2 as well, use the old style BUG()
statement if compiled with older gcc versions.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>