gdb: int to bool in struct entry_info

Convert 'struct entry_info' to use bool for flag fields.  The places
where these flags are set are updated too.  I have also removed the
bit width specifier ": 1" from the flag field definitions.  There is
one entry_info struct per BFD in GDB, which is not a huge number, so
forcing the bool fields to 1-bit doesn't seen necessary.

There should be no user visible changes after this commit.

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