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.
Are there future plans to expand this system call for other purposes?
If not, may I suggest more descriptive names, e.g., `__NR_riscv_atomic` and `sys_riscv_atomic`?