arch-arm: Disambuiguate NumFloatV7ArchRegs usage

arch-arm: Disambuiguate NumFloatV7ArchRegs usage

Sometimes NumFloatV7ArchRegs is used to specify the maximum number of
AArch32 floating point registers. Sometimes it is just used for indexing
a free register storage to be used by microcode.  In that scenario,
VecSpecialElem should be used, which is a index to the first available
non architectural register for floating point.

Change-Id: I4e84740701f0e7041cf1acad2afed471361c423a
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/23107
Tested-by: kokoro <noreply+kokoro@google.com>