gdb: Revert FAKE_PROCESS id code to prepare for gdb 7.6.1

gdb: Revert FAKE_PROCESS id code to prepare for gdb 7.6.1

This set of local modifications to support the swapper as an inferior
for kgdb is being reverted to ease the merge of the new gdb 7.6.1 code.
However, it appears that gdb 7.6.1 has implemented the concept of a
"fake" process ID independently, so it may not be necessary to add
these modifications back later.