This patch allows padding to be specified per-target for libcalls. This
hasn't been traditionally important, because libcalls haven't accepted
quantities which might need padding, but that's no longer true with the
new(-ish) fixed-point support helper functions.

Tested (alongside other fixed-point support patches) with cross to ARM
EABI in both big & little-endian mode (the target-specific part is to
avoid a behaviour change for half-float types on ARM). OK to apply?

Thanks,

Julian

ChangeLog

    gcc/
    * calls.c (emit_library_call_value_1): Support padding for libcall
    arguments and return values.
    * config/arm/arm.c (arm_pad_arg_upward): Pad half-float values
    downwards in big-endian mode.