Merging PCH to mainline

[David Edelsohn]

	I have tracked down another PCH-related rs6000 failure and I can
fix it by adding GTY(()), but I would have thought that the logic is
correct now and adding PCH machinery wrong.

	The bug is when compiling with PCH, GCC emits the assembler extern
declarations of the AIX common-mode functions for each function compiled
instead of once with the first function in the file.  Emitting declaration
is controlled by a static variable common_mode_defined in rs6000.c.
Adding GTY(()) to the declaration produces correct behavior, but this
should not be information associated with PCH, so I do not understand why
restoring its state should be correct.  It seems that common_mode_defined
would need to be reset for each function for things to be working
incorrectly now.

David