MIPS: signal: Remove FP context support when CONFIG_MIPS_FP_SUPPORT=n

When CONFIG_MIPS_FP_SUPPORT=n we don't support floating point, so
there's no need to save & restore floating point context around signals.
This prepares us for the removal of FP context from struct task_struct
later.

Since MSA context is a superset of FP context support for it similarly
needs to be removed when MSA/FP support is disabled.

Signed-off-by: Paul Burton <paul.burton@mips.com>
Patchwork: https://patchwork.linux-mips.org/patch/21009/
Cc: linux-mips@linux-mips.org