kselftest/arm64: Remove redundant _start labels from zt-test

kselftest/arm64: Remove redundant _start labels from zt-test

The newly added zt-test program copied the pattern from the other FP
stress test programs of having a redundant _start label which is
rejected by clang, as we did in a parallel series for the other tests
remove the label so we can build with clang.

No functional change.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20230130-arm64-fix-sme2-clang-v1-1-3ce81d99ea8f@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>