[MCJIT] Add options to llvm-rtdyld to describe a phony target address space for
use in -verify mode.

This patch adds three hidden command line options to llvm-rtdyld:

 -target-addr-start <start-addr> : Specify the start of the virtual address
                                   space on the phony target.

 -target-addr-end   <end-addr>   : Specify the end of the virtual address space
                                   on the phony target.

 -target-section-sep <sep>       : Specify the separation (in bytes) between the
                                   end of one section and the start of the next.

These options automatically default to sane values for the target platform. In
particular, they allow narrow (e.g. 32-bit, 16-bit) targets to be tested from
wider (e.g. 64-bit, 32-bit) hosts without overflowing pointers.

The section separation option defaults to zero, but can be set to a large number
(e.g. 1 << 32) to force large separations between sections in order to
stress-test large-code-model code.