gdb: fix start breakpoint expression not working in some languages

gdb: fix start breakpoint expression not working in some languages

Commit 0be837be9fb4 ("gdb: make "start" breakpoint inferior-specific")
regresses gdb.ada/start.exp:

    (gdb) start
    Error in expression, near `1'.
    (gdb) UNTESTED: gdb.ada/start.exp: start failed to land inside the right procedure

This is because in Ada, the equality operator is =, not ==.

I checked the other languages supported by GDB, these other languages
use = for equality:

 - Pascal: tests like gdb.pascal/hello.exp are affected too
 - Modula-2: I tried building a Modula-2 hello world using gm2, but it
   seems like the generated DWARF doesn't specify the Modula-2 language
   in the CUs, it's C++ and C, so the selected language isn't
   "modula-2".  But if I manually do "set language modula-2" on a dummy
   program and then "start", I get the same error.

Other languages all use ==.

So, a short term fix would be to use = or == in the expression, based on
the current language.  If this was meant to be permanent, I would
suggest adding something like an "equality_operator" method to
language_defn, that returns the right equality operator for the
language.  But the goal is to replace all this with proper
inferior-specific breakpoints, so I hope all this is temporary.

Approved-By: Tom de Vries <tdevries@suse.de>
Change-Id: Id4d38e14a80e6bbbb1ad2b2277f974dd55192969