gdb/dwarf: add some QUIT macros

While testing the fix for PR 29105, I noticed I couldn't ctrl-C my way
out of GDB expanding many symtabs.  GDB was busy in a loop in
cooked_index_functions::expand_symtabs_matching.  Add a QUIT there.  I
also happened to see a spot in
cooked_index_functions::expand_matching_symbols where a QUIT would be
useful too, since we iterate over a potentially big number of index
entries and expand CUs in the loop.  Add one there too.

Change-Id: Ie1d650381df7f944c16d841b3e592d2dce7306c3
Approved-By: Kevin Buettner <kevinb@redhat.com>