mips: sync with GNU libc, fix mips64r6 n32 compilation

libc/sysdeps/linux/mips/sys/asm.h

@@ -415,12 +415,19 @@ symbol		=	value
 #if _MIPS_SIM == _MIPS_SIM_NABI32
 # define PTR_ADD	add
 # define PTR_ADDI	addi
-# define PTR_ADDU	add /* no u */
-# define PTR_ADDIU	addi /* no u */
 # define PTR_SUB	sub
 # define PTR_SUBI	subi
+#if !defined __mips_isa_rev || __mips_isa_rev < 6
+# define PTR_ADDU	add /* no u */
+# define PTR_ADDIU	addi /* no u */
 # define PTR_SUBU	sub /* no u */
 # define PTR_SUBIU	sub /* no u */
+#else
+# define PTR_ADDU       addu
+# define PTR_ADDIU      addiu
+# define PTR_SUBU       subu
+# define PTR_SUBIU      subu
+#endif
 # define PTR_L		lw
 # define PTR_LA		la
 # define PTR_S		sw