powerpc/signal: Update comment for clarity

The comment being referred to was deleted in commit af1bbc3d ("powerpc:
Remove UP only lazy floating point and vector optimisations").

Add a bit more detail so it's clear why we need to clear the FP/VEC/VSX
bits here.

Signed-off-by: Rashmica Gupta <rashmica@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220617043135.426897-1-rashmica@linux.ibm.com