Using global variable in xnu kernel, set # of addressable bits

The kernel has a global variable with the TCR_EL1.T1SZ value,
from which was can calculate the number of addressable bits.
Find that symbol in DynamicLoaderDarwinKernel and set the bits
to that value for this Process.

Differential Revision: https://reviews.llvm.org/D147462
rdar://107445318