[lit] Use ld.lld -z separate-code to work around a debug_line parsing bug

The issue was exposed by D64903/r367537.
http://lab.llvm.org:8011/builders/lldb-x64-windows-ninja/builds/7321/

In these tests, .debug_str immediately follows .text.
The last section of last RX PT_LOAD was originally padded with trap
instructions and .debug_str started at a new page (actually
common-page-size). Now, .debug_str immediately follows .test.
Add -z separate-code to use the old layout.

llvm-svn: 367549