[LLD][ELF] Fix compressed-debug-level test on SystemZ

The libz compression library on SystemZ by default makes use of the
platform's hardware-accelerated compression facility. This is much
faster than the regular software implementation, but often results in
slightly different outputs. This causes failures with the
compressed-debug-level test case.

To fix this, run this test while setting the DFLTCC environment
variable to zero, which prevents use of hardware compression and falls
back to the software implementation. (This should not have any effect
on other platforms.)

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D149273