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

Post-commit review pointed out a few style issues in commit 8b9c08eddac
("[gdb/symtab] Add name_of_main and language_of_main to the DWARF index").

Fix these.

Tested on x86_64-linux.

Reported-By: Tom Tromey <tom@tromey.com>
Approved-By: Tom Tromey <tom@tromey.com>