Remove defective atomics implementations

The AArch64 and RISC-V atomic implementations are defective, remove them
and replace them with the generic GCC atomic builtins.

Fixes #247

Author: andreas-schwab

include/ck_pr.h

@@ -62,10 +62,6 @@
 #include "gcc/ppc/ck_pr.h"
 #elif defined(__arm__)
 #include "gcc/arm/ck_pr.h"
-#elif defined(__aarch64__)
-#include "gcc/aarch64/ck_pr.h"
-#elif defined(__riscv) && __riscv_xlen == 64
-#include "gcc/riscv64/ck_pr.h"
 #elif !defined(__GNUC__)
 #error Your platform is unsupported
 #endif