arch-arm: SVE instructions do not use AHP format

arch-arm: SVE instructions do not use AHP format

SVE half-precision floating-point instructions support only IEEE
754-2008 half-precision format and ignore the value of the FPCR.AHP bit,
behaving as if it has an Effective value of 0.

This patch is addressing this by masking the FPSCR.AHB bit before
passing it to fplib.

Change-Id: I1432fc3f7fefb81445fe042ae7d681f5cec40e64
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/+/28108
Tested-by: kokoro <noreply+kokoro@google.com>