MIPS: Predefine __mips_strict_alignment if STRICT_ALIGNMENT

Arm32 predefines __ARM_FEATURE_UNALIGNED if -mno-unaligned-access,
and RISC-V predefines __riscv_misaligned_avoid.

Let's define __mips_strict_alignment for MIPSr6 and -mstrict-align
is used.

Note that, this macro is always defined for pre-R6.

gcc
	* config/mips/mips.h (TARGET_CPU_CPP_BUILTINS): Predefine
	__mips_strict_alignment if STRICT_ALIGNMENT.

gcc/config/mips/mips.h

@@ -694,6 +694,9 @@ struct mips_cpu_info {
 	builtin_define ("__mips_compact_branches_always");		\
       else 								\
 	builtin_define ("__mips_compact_branches_optimal");		\
+									\
+      if (STRICT_ALIGNMENT)						\
+	builtin_define ("__mips_strict_alignment");			\
     }									\
   while (0)