Remove gcc/nios2.h and gcc/riscv.h files, include gcc/generic.h directly

Remove gcc/nios2.h and gcc/riscv.h files, include gcc/generic.h directly
(code refactoring)

* src/Makefile.am (nobase_private_HEADERS): Remove nios2.h, riscv.h
entries.
* src/atomic_ops.h [__GNUC__ && !AO_USE_PTHREAD_DEFS
&& !__INTEL_COMPILER]: Use #elif instead of #endif - #if pairs.
* src/atomic_ops.h [__GNUC__ && !AO_USE_PTHREAD_DEFS
&& !__INTEL_COMPILER && (__nios2__ || __riscv)]: Do not include
gcc/nios2.h and gcc/riscv.h files; gcc/generic.h is included instead.
* src/atomic_ops/sysdeps/gcc/nios2.h: Delete file.
* src/atomic_ops/sysdeps/gcc/riscv.h: Likewise.