arm64: Drop workaround for broken 'S' constraint with GCC 4.9

arm64: Drop workaround for broken 'S' constraint with GCC 4.9

Since GCC < 5.1 has been shown to be unsuitable for the arm64 kernel,
let's drop the workaround for the 'S' asm constraint that GCC 4.9
doesn't always grok.

This is effectively a revert of 9fd339a45be5 ("arm64: Work around
broken GCC 4.9 handling of "S" constraint").

Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210118130129.2875949-1-maz@kernel.org
Signed-off-by: Will Deacon <will@kernel.org>