getMainExecutable: Fix hand-rolled AT_EXECPATH for older FreeBSD
Once we hit AT_NULL, we need to bail out of the loop; not just the
enclosing switch. This fixes basic usage (e.g. `cc --version`) when
AT_EXECPATH isn't present on older branches (e.g. under
emu-user-static, at the moment), where we would previously run off
the end of ::environ.
Patch By: kevans
Reviewed By: arichardson
Differential Revision: https://reviews.llvm.org/D79239
MFC after: 3 days
c7d9dfe810