MIPS: Fix delay slot bug in `atomic*_sub_if_positive' for R10000_LLSC_WAR

This patch fixes an old bug in MIPS ll/sc atomics, in the
`atomic_sub_if_positive' and `atomic64_sub_if_positive' functions, for
the R10000_LLSC_WAR case where the result of the subu/dsubu instruction
would potentially not be made available to the sc/scd instruction due
to being in the delay-slot of the branch-likely (beqzl) instruction.

This also removes the need for the `noreorder' directive, allowing GAS
to use delay slot scheduling as needed.

The same fix is also applied to the standard branch (beqz) case in
preparation for a follow-up patch that will cleanup/merge the
R10000_LLSC_WAR and non-R10K sections together.

Signed-off-by: Joshua Kinard <kumba@gentoo.org>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Tested-by: Joshua Kinard <kumba@gentoo.org>
Patchwork: https://patchwork.linux-mips.org/patch/17735/
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: James Hogan <james.hogan@mips.com>
Cc: "Maciej W. Rozycki" <macro@mips.com>
Cc: linux-mips@linux-mips.org