kselftest/arm64: Fix build with stricter assemblers

kselftest/arm64: Fix build with stricter assemblers

While some assemblers (including the LLVM assembler I mostly use) will
happily accept SMSTART as an instruction by default others, specifically
gas, require that any architecture extensions be explicitly enabled.
The assembler SME test programs use manually encoded helpers for the new
instructions but no SMSTART helper is defined, only SM and ZA specific
variants.  Unfortunately the irritators that were just added use plain
SMSTART so on stricter assemblers these fail to build:

za-test.S:160: Error: selected processor does not support `smstart'

Switch to using SMSTART ZA via the manually encoded smstart_za macro we
already have defined.

Fixes: d65f27d240bb ("kselftest/arm64: Implement irritators for ZA and ZT")
Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20241108-arm64-selftest-asm-error-v1-1-7ce27b42a677@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>