[libc][rpc] Update locking to work on volta

Carefully work around not knowing the thread mask that nvptx intrinsic
functions require.

If the warp is converged when calling try_lock, a single rpc call will handle
all lanes within it. Otherwise more than one rpc call with thread masks that
compose to the unknown one will occur.

Reviewed By: jhuber6

Differential Revision: https://reviews.llvm.org/D149897