MIPS: Cleanup R10000_LLSC_WAR logic in atomic.h

This patch reduces down the conditionals in MIPS atomic code that deal
with a silicon bug in early R10000 cpus that required a workaround of
a branch-likely instruction following a store-conditional in order to
to guarantee the whole ll/sc sequence is atomic.  As the only real
difference is a branch-likely instruction (beqzl) over a standard
branch (beqz), the conditional is reduced down to a single preprocessor
check at the top to pick the required instruction.

This requires writing the uses in assembler, thus we discard the
non-R10000 case that uses a mixture of a C do...while loop with
embedded assembler that was added back in commit 7837314d141c ("MIPS:
Get rid of branches to .subsections.").  A note found in the git log
for commit 5999eca25c1f ("[MIPS] Improve branch prediction in ll/sc
atomic operations.") is also addressed.

The macro definition for the branch instruction and the code comment
derives from a patch sent in earlier by Paul Burton for various cmpxchg
cleanups.

[[email protected]:
  - Minor whitespace fix for checkpatch.]

Signed-off-by: Joshua Kinard <[email protected]>
Signed-off-by: Paul Burton <[email protected]>
Patchwork: https://patchwork.linux-mips.org/patch/17736/
Cc: Ralf Baechle <[email protected]>
Cc: James Hogan <[email protected]>
Cc: "Maciej W. Rozycki" <[email protected]>
Cc: [email protected]

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