add clang-compatible thread-pointer code for mips

clang does not presently support the "v" constraint we want to use to
get the result from $3, and trying to use register...__asm__("$3") to
do the same invokes serious compiler bugs. so for now, i'm working
around the issue with an extra temp register and putting $3 in the
clobber list instead of using it as output. when the bugs in clang are
fixed, this issue should be revisited to generate smaller/faster code
like what gcc gets.

arch/mips/pthread_arch.h

 static inline struct pthread *__pthread_self()
 {
 	struct pthread *self;
+#ifdef __clang__
+	__asm__ __volatile__ (".word 0x7c03e83b ; move %0, $3" : "=r" (self) : : "$3" );
+#else
 	__asm__ __volatile__ (".word 0x7c03e83b" : "=v" (self) );
+#endif
 	return self;
 }