[gdb/symtab] Fix more style issues in v9 .gdb_index section support

I noticed a few more style issues in commit 8b9c08eddac ("[gdb/symtab] Add
name_of_main and language_of_main to the DWARF index"), after checking it
with gcc's check_GNU_style.{sh,py}.

Fix these.

Build on x86_64-linux.