gdb: Improve concept of FAKE_PROCESS_ID to fix JIT crash

gdb: Improve concept of FAKE_PROCESS_ID to fix JIT crash

The fix for 7.2 seems to break on gdb 7.3 with addition of JIT
initialization which assumes at least one inferior defined.  The problem
is that this inferior isn't defined before the JIT routine is run.

Changing the fake process ID from zero has a lot of risk (miss some,
accidently change zero's that need to remain zero, etc), but it seems to
be the only way to fix it cleanly.