opcodes: fix -std=gnu23 compatibility wrt static_assert

opcodes: fix -std=gnu23 compatibility wrt static_assert

static_assert is declared in C23 so we can't reuse that identifier:
* Define our own static_assert conditionally;

* Rename "static assert" hacks to _N as we do already in some places
  to avoid a conflict.

ChangeLog:
	PR ld/32372

        * i386-gen.c (static_assert): Define conditionally.
        * mips-formats.h (MAPPED_INT): Rename identifier.
        (MAPPED_REG): Rename identifier.
        (OPTIONAL_MAPPED_REG): Rename identifier.
        * s390-opc.c (static_assert): Define conditionally.