Function process_builder::compute_initial_stack_pointer

pub fn compute_initial_stack_pointer(base: usize, size: usize) -> usize

Given the base and size of the stack block, compute the appropriate initial SP value for an initial thread according to the C calling convention for the machine.

Copied from, and must be kept in sync with: //zircon/system/ulib/elf-psabi/include/lib/elf-psabi/sp.h