riscv: Only send remote fences when some other CPU is online

If no other CPU is online, a local cache or TLB flush is sufficient.
These checks can be constant-folded when SMP is disabled.

Signed-off-by: Samuel Holland <samuel.holland@sifive.com>
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/r/20240327045035.368512-6-samuel.holland@sifive.com


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>