mpi/ec-inline: reduce register pressure on 32-bit ARM

* mpi/ec-inline.h [HAVE_COMPATIBLE_GCC_ARM_PLATFORM_AS] (ADD4_LIMB32)
(ADD6_LIMB32, SUB4_LIMB32, SUB6_LIMB32): Reuse input registers
as output (use just two unique operators).

This fixes building ec-nist.c with GCC-14 on 32-bit ARM.

• GnuPG-bug-id: T7226
• Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>