[arm] Use -fno-short-enums for all 32-bit ARM builds

[arm] Use -fno-short-enums for all 32-bit ARM builds

The EFI ABI requires the use of -fno-short-enums, and the EDK2 headers
will perform a compile-time check that enums are 32 bits.

The EDK2 headers may be included even in builds for non-EFI platforms,
and so the -fno-short-enums flag must be used in all 32-bit ARM
builds.  Fortunately, nothing else currently cares about enum sizes.

Signed-off-by: Michael Brown <mcb30@ipxe.org>