libgcc: Small bitint_reduce_prec big-endian fixes

libgcc: Small bitint_reduce_prec big-endian fixes

The big-endian _BitInt support in libgcc was written without any
testing and so I haven't discovered I've made one mistake in it
(in multiple places).
The bitint_reduce_prec function attempts to optimize inputs
which have some larger precision but at runtime they are found
to need smaller number of limbs.
For little-endian that is handled just by returning smaller
precision (or negative precision for signed), but for
big-endian we need to adjust the passed in limb pointer so that
when it returns smaller precision the argument still contains
the least significant limbs for the returned precision.

2025-05-20  Jakub Jelinek  <jakub@redhat.com>

	* libgcc2.c (bitint_reduce_prec): For big endian
	__LIBGCC_BITINT_ORDER__ use ++*p and --*p instead of
	++p and --p.
	* soft-fp/bitint.h (bitint_reduce_prec): Likewise.