[libc++abi] Fix remote execution of .sh.cpp tests

This aims to fix test failures on the following buildbots:

- http://lab.llvm.org:8011/builders/llvm-clang-win-x-armv7l
- http://lab.llvm.org:8011/builders/llvm-clang-win-x-aarch64

Differential Revision: https://reviews.llvm.org/D77190