Restore definitions of static members in MklCpuAllocator.

These were removed in #17396 which made the static member variables of
MklCpuAllocator into inline variables, which are a C++17 feature, and not
properly restored in #18006 which reverted the inline declarations, leading to
an ODR violation that is apparently ignored with some compilers.
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RELNOTES: n/a

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Automated g4 rollback of changelist 191305220

PiperOrigin-RevId: 192836808