[mips] flip from =v to =r - let the compiler choose a temp register.

=v is some ye olde gcc "use this specific register as the temp register"
thing that they've deprecated and clang/llvm doesn't implement.

Poked again and again by: sbruno

libexec/rtld-elf/mips/reloc.c

@@ -645,7 +645,7 @@ _mips_get_tls(void)
 	    ".set\tmips64r2\n\t"
 	    "rdhwr\t%0, $29\n\t"
 	    ".set\tpop"
-	    : "=v" (_rv));
+	    : "=r" (_rv));
 	/*
 	 * XXXSS See 'git show c6be4f4d2d1b71c04de5d3bbb6933ce2dbcdb317'
 	 *
@@ -670,7 +670,7 @@ _mips_get_tls(void)
 	    ".set\tmips32r2\n\t"
 	    "rdhwr\t%0, $29\n\t"
 	    ".set\tpop"
-	    : "=v" (_rv));
+	    : "=r" (_rv));
 	/*
 	 * XXXSS See 'git show c6be4f4d2d1b71c04de5d3bbb6933ce2dbcdb317'
 	 *