[TSAN][Darwin] Forward declare spinlock functions on darwin for TSAN interceptors

Spinlock symbols are removed from headers in MacOS version 10.12 and greater.
Even though they are deprecated, the symbols remain available on the system.

The TSAN interceptors currently cause a build failure after this version because
of the change in availability of the symbol.

We want to continue intercepting the symbols available on the OS.
So we add forward declarations so that the TSAN interceptors can build.

This is tested with the existing osspinlock_norace test.

Differential Revision: https://reviews.llvm.org/D146537