arm: Fix clang error in sve_vl()

Fix the following clang error in sve_vl():

In file included from arm/selftest.c:16:
kvm-unit-tests/lib/asm/processor.h:163:16:
error: value size does not match register size specified by the
constraint and modifier [-Werror,-Wasm-operand-widths]
                     : "=r" (vl));
                             ^
kvm-unit-tests/lib/asm/processor.h:162:14:
note: use constraint modifier "w"
                     "rdvl %0, #8"
                           ^~
                           %w0
1 error generated.

Fixes: d47d370c ("arm: Add test for FPU/SIMD context save/restore")
Signed-off-by: Raghavendra Rao Ananta <rananta@google.com>
Signed-off-by: Andrew Jones <andrew.jones@linux.dev>