libgcc: Fix COPY_ARG_VAL initializer (PR 117537)

We recently forced -Werror when building libgcc for aarch64, to make
sure we'd catch and fix the kind of problem described in the PR.

In this case, when building for aarch64_be (so, big endian), gcc emits
this warning/error:
libgcc/config/libbid/bid_conf.h:847:25: error: missing braces around initializer [-Werror=missing-braces]
  847 |        UINT128 arg_name={ bid_##arg_name.w[1], bid_##arg_name.w[0]};
libgcc/config/libbid/bid_conf.h:871:8: note: in expansion of macro 'COPY_ARG_VAL'
  871 |        COPY_ARG_VAL(arg_name)

This patch fixes the problem by adding curly braces around the
initializer for COPY_ARG_VAL in the big endian case.

It seems that COPY_ARG_REF (just above COPY_ARG_VAL) has a similar
issue, but DECIMAL_CALL_BY_REFERENCE seems always defined to 0, so
COPY_ARG_REF is never used.  The patch fixes it too, though.

libgcc/config/libbid/ChangeLog:

	PR libgcc/117537
	* bid_conf.h (COPY_ARG_REF): Fix initializer.
	(COPY_ARG_VAL): Likewise.