Change the size argument of __getcwd() to size_t.

This matches the getcwd() definition.

This is technically an ABI change, but that would only effect 64-bit
big-endian platforms that pass arguments on the stack. We have none of
those.

Reviewed by: jhb
Obtained from: CheriABI
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D9428