arm64: Use test_tsk_thread_flag() for checking TIF_SINGLESTEP

[ Upstream commit 5afc78551bf5d53279036e0bf63314e35631d79f ]

Rather than open-code test_tsk_thread_flag() at each callsite, simply
replace the couple of offenders with calls to test_tsk_thread_flag()
directly.

Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>