MFC r311169,r311898,r312925,r312973,r312975,r313007,r313040,r313080,

MFC r311169,r311898,r312925,r312973,r312975,r313007,r313040,r313080,
    r313254,r313341

amd64: add atomic_fcmpset

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sparc64: add atomic_fcmpset

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Implement atomic_fcmpset_* for arm and arm64.

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Add atomic_fcmpset_*() inlines for powerpc

Summary:
atomic_fcmpset_*() is analogous to atomic_cmpset(), but saves off the read value
from the target memory location into the 'old' pointer in the case of failure.

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i386: add atomic_fcmpset

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Don't retry a lost reservation in atomic_fcmpset()

The desired behavior of atomic_fcmpset_() is to always exit on error.  Instead
of retrying on lost reservation, leave the retry to the caller, and return

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Add atomic_fcmpset_*() inlines for MIPS

atomic_fcmpset_*() is analogous to atomic_cmpset(), but saves off the
read value from the target memory location into the 'old' pointer.

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i386: fixup fcmpset

An incorrect output specifier was used which worked with clang by accident,
but breaks with the in-tree gcc version.

While here plug a whitespace nit.

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Implement atomic_fcmpset_*() for RISC-V.

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Use 64bit store instruction in atomic_fcmpset_64.