genksyms: fix metag symbol prefix on crc symbols

Meta uses symbol prefixes, so add "metag" to the list of architectures
to set the mod_prefix to "_" for. This fixes __crc_* symbols to add the
extra underscore to match _CRC_SYMBOL macro in <linux/export.h> and so
that modpost finds them.

Signed-off-by: James Hogan <james.hogan@imgtec.com>