arm: Do not process rest of MVE header file after unsupported error

arm: Do not process rest of MVE header file after unsupported error

This patch makes sure the rest of the header file is not parsed if MVE is not
supported.  The user should not be including this file if MVE is not supported,
nevertheless making sure it doesn't parse the rest of the header file will
save the user from a huge error output that would be rather useless.

gcc/ChangeLog:
2020-04-03  Andre Vieira  <andre.simoesdiasvieira@arm.com>

        * config/arm/arm_mve.h: Condition the header file on __ARM_FEATURE_MVE.