mips: MIPS backend, meet C++20

mips: MIPS backend, meet C++20

C++20, in particular https://wg21.link/P1120R0 paper voted into it,
deprecates various operations between enumerators from different enumeration
types etc., and as we've switched to -std=gnu++20 by default, this now
results in warnings or errors during stage2 and onwards.

The following patch should fix mips build.

2025-11-28  Jakub Jelinek  <jakub@redhat.com>

	* config/mips/mips.cc (mips_unspec_address_offset): Avoid
	arithmetics between enumerators from different enum types.