[fusl] Remove code for unsupported architectures

fusl/arch/mips/atomic_arch.h

-#define a_ll a_ll
-static inline int a_ll(volatile int* p) {
-  int v;
-  __asm__ __volatile__(
-      ".set push ; .set mips2\n\t"
-      "ll %0, %1"
-      "\n\t.set pop"
-      : "=r"(v)
-      : "m"(*p));
-  return v;
-}
-
-#define a_sc a_sc
-static inline int a_sc(volatile int* p, int v) {
-  int r;
-  __asm__ __volatile__(
-      ".set push ; .set mips2\n\t"
-      "sc %0, %1"
-      "\n\t.set pop"
-      : "=r"(r), "=m"(*p)
-      : "0"(v)
-      : "memory");
-  return r;
-}
-
-#define a_barrier a_barrier
-static inline void a_barrier() {
-  /* mips2 sync, but using too many directives causes
-   * gcc not to inline it, so encode with .long instead. */
-  __asm__ __volatile__(".long 0xf" : : : "memory");
-}
-
-#define a_pre_llsc a_barrier
-#define a_post_llsc a_barrier