Fix MIPS build.

Although 'register' is deprecated, we need to use v1, and there's
no way to do that through register constraints on the assembler
fragment itself.

Change-Id: Ib5b12c4c3652513d10cc61d4a4b11314ece25663

libc/private/bionic_tls.h

@@ -95,7 +95,7 @@ extern int __set_tls(void* ptr);
 #elif defined(__mips__)
 # define __get_tls() \
     /* On mips32r1, this goes via a kernel illegal instruction trap that's optimized for v1. */ \
-    ({ unsigned int __val asm("v1"); \
+    ({ register unsigned int __val asm("v1"); \
        asm ("   .set    push\n" \
             "   .set    mips32r2\n" \
             "   rdhwr   %0,$29\n" \