Add the ability to debug through an exec into ld

A previous commit enabled LLDB to be able to debug a program launched via ld: https://reviews.llvm.org/D108061.

This commit adds the ability to debug a program launched via ld when it happens during an exec into the dynamic loader. There was an issue where after the exec we would locate the rendezvous structure right away but it didn't contain any valid values and we would try to set the dyanamic loader breakpoint at address zero. This patch fixes that and adds a test.

Differential Revision: https://reviews.llvm.org/D125253