RISC-V: always select RISCV_ALTERNATIVE for non-xip kernels

RISC-V: always select RISCV_ALTERNATIVE for non-xip kernels

When moving switch_to's has_fpu() over to using
riscv_has_extension_likely() rather than static branches, the FPU code
gained a dependency on the alternatives framework.
That dependency has now been removed, as riscv_has_extension_ikely() now
contains a fallback path, using __riscv_isa_extension_available(), but
if CONFIG_RISCV_ALTERNATIVE isn't selected when CONFIG_FPU is, has_fpu()
checks will not benefit from the "fast path" that the alternatives
framework provides.

We want to ensure that alternatives are available whenever
riscv_has_extension_[un]likely() is used, rather than silently falling
back to the slow path, but rather than rely on selecting
RISCV_ALTERNATIVE in the myriad of locations that may use
riscv_has_extension_[un]likely(), select it (almost) always instead by
adding it to the main RISCV config entry.
xip kernels cannot make use of the alternatives framework, so it is not
enabled for those configurations, although this is the status quo.

All current sites that select RISCV_ALTERNATIVE are converted to
dependencies on the option instead. The explicit dependencies on
!XIP_KERNEL can be dropped, as RISCV_ALTERNATIVE is not user selectable.

Fixes: 702e64550b12 ("riscv: fpu: switch has_fpu() to riscv_has_extension_likely()")
Link: https://lore.kernel.org/all/ZBruFRwt3rUVngPu@zx2c4.com/
Reported-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Jason A. Donenfeld <Jason@zx2c4.com>
Link: https://lore.kernel.org/r/20230324100538.3514663-3-conor.dooley@microchip.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>