Use uintptr_t instead of register_t * for the stack base.

• Use ustringp for the location of the argv and environment strings and allow destp to travel further down the stack for the stackgap and auxv regions.
• Update the Linux copyout_strings variants to move destp down the stack as was done for the native ABIs in r263349.
• Stop allocating a space for a stack gap in the Linux ABIs. This used to hold translated system call arguments, but hasn't been used since r159992.

Reviewed by: kib
Tested on: md64 (amd64, i386, linux64), i386 (i386, linux)
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D22501