mipsel (little endian) support

the fields in the mcontext_t are long long (for no good reason) even
on 32-bit mips, so the offset of the instruction pointer (as a word)
varies depending on endianness.

arch/mips/pthread_arch.h

@@ -5,4 +5,4 @@ static inline struct pthread *__pthread_self()
 	return self;
 }
 
-#define CANCEL_REG_IP 3
+#define CANCEL_REG_IP (3-(union {int __i; char __b;}){1}.__b)