[Driver] Clean up -gz & --compress-debug-sections

* Make cc1 and cc1as --compress-debug-sections an alias for --compress-debug-sections=zlib
* Make -gz an alias for -gz=zlib

The new behavior is consistent with GCC when binutils>=2.26 is detected:
-gz is translated to --compress-debug-sections=zlib instead of --compress-debug-sections.