Improve atomic memory access and support -mno-atomic.

Use CONFIG_GENERIC_ATOMIC64 if 32-bit.  Add system call to emulate
atomic compare exchange, primarily for targets lacking hardware
support (i.e., lacking RV32A, RV64A).  Support excluding use of
atomic memory instructions (-mno-atomic).

This change is mutually dependent on the corresponding change to
riscv-gcc.
The inline assembly appears equivalent to `__cmpxchg_local_generic()` as defined in `<asm-generic/cmpxchg-local.h>`.  Is there a particular reason to avoid the generic definition of `cmpxchg_local()`?