[fusl] Remove code for unsupported architectures

fusl/arch/powerpc/atomic_arch.h

-#define a_ll a_ll
-static inline int a_ll(volatile int* p) {
-  int v;
-  __asm__ __volatile__("lwarx %0, 0, %2" : "=r"(v) : "m"(*p), "r"(p));
-  return v;
-}
-
-#define a_sc a_sc
-static inline int a_sc(volatile int* p, int v) {
-  int r;
-  __asm__ __volatile__("stwcx. %2, 0, %3 ; mfcr %0"
-                       : "=r"(r), "=m"(*p)
-                       : "r"(v), "r"(p)
-                       : "memory", "cc");
-  return r & 0x20000000; /* "bit 2" of "cr0" (backwards bit order) */
-}
-
-#define a_barrier a_barrier
-static inline void a_barrier() {
-  __asm__ __volatile__("sync" : : : "memory");
-}
-
-#define a_pre_llsc a_barrier
-
-#define a_post_llsc a_post_llsc
-static inline void a_post_llsc() {
-  __asm__ __volatile__("isync" : : : "memory");
-}
-
-#define a_store a_store
-static inline void a_store(volatile int* p, int v) {
-  a_pre_llsc();
-  *p = v;
-  a_post_llsc();
-}