[MIPS] Allow united mipsel and mips64el gcc toolchain

Explicitly tell 32-bit links that they are doing 32-bit links.
This is needed when using united 32-bit and 64-bit toolchains.
This is harmless when using older separate 32-only toolchains.

Change-Id: I70cbd3f5867e59b1f6f829793444242fb0894aa6
diff --git a/libc/arch-mips/mips.mk b/libc/arch-mips/mips.mk
index 0a3718b..5705c8a 100644
--- a/libc/arch-mips/mips.mk
+++ b/libc/arch-mips/mips.mk
@@ -83,3 +83,6 @@
 
 libc_crt_target_so_cflags_mips := \
     -fPIC
+
+libc_crt_target_ldflags_mips := \
+    -melf32ltsmip