[llvm-dwarfdump] - Cleanup of gnu_call_site.s. NFC.

This change fixes values of test so that it passes 
-verify without errors and also adds comments.
Test was introduced in D39119 and intention was to check
that tool is able to dump few
DW_*GNU_call_site* tags and attributes, so that
change is NFC cleanup.

llvm-svn: 316428