MIPS32: Do not use _MIPS_ARCH_MIPS64R6 in MIPS32 code

Test: booted MIPS32 and MIPS64 in QEMU

Change-Id: I413272c11ad00a0f77271302727b5860eaa7756b
diff --git a/runtime/arch/mips/quick_entrypoints_mips.S b/runtime/arch/mips/quick_entrypoints_mips.S
index ec8ae85..4f7b495 100644
--- a/runtime/arch/mips/quick_entrypoints_mips.S
+++ b/runtime/arch/mips/quick_entrypoints_mips.S
@@ -2048,11 +2048,12 @@
     lw    $t0, MIRROR_STRING_COUNT_OFFSET($a0)    # this.length()
 #endif
     slt   $t1, $a2, $zero # if fromIndex < 0
-#if defined(_MIPS_ARCH_MIPS32R6) || defined(_MIPS_ARCH_MIPS64R6)
+#if defined(_MIPS_ARCH_MIPS32R6)
     seleqz $a2, $a2, $t1  #     fromIndex = 0;
 #else
     movn   $a2, $zero, $t1 #    fromIndex = 0;
 #endif
+
 #if (STRING_COMPRESSION_FEATURE)
     srl   $t0, $a3, 1     # $a3 holds count (with flag) and $t0 holds actual length
 #endif