arch-arm: Fix NumVecV7ArchRegs value (64->16)

arch-arm: Fix NumVecV7ArchRegs value (64->16)

In armv7 there are 16 only quadword (vector) registers which are usable
by SIMD instructions (Q0-Q15). Those completely overlap with the 32
double word registers (D0-D31).

NumVecV7ArchRegs  = 16; // Q0-Q15

Change-Id: Id8fee1064d60dcfa54f273fa7d579a20c0087835
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/+/23105
Tested-by: kokoro <noreply+kokoro@google.com>