RISC-V: Use the extension probing code to enable the FPU

This pull the static key management code for the FPU into the generic
ISA extension probing code, so it can be used by other extensions that
need static keys.

* 'riscv-static_key' of ssh://gitolite.kernel.org/pub/scm/linux/kernel/git/palmer/linux:
  riscv: switch has_fpu() to the unified static key mechanism
  riscv: introduce unified static key mechanism for ISA extensions