[Cell/B.E.] Fix regression due to gdbarch_significant_addr_bit

[Cell/B.E.] Fix regression due to gdbarch_significant_addr_bit

On Cell/B.E. multi-architecture debugging we use a "merged" address space
that encodes both the main PowerPC address space and the local store address
spaces of all active SPUs.  This will always occupy 64 bits.

However, gdbarch_addr_bit is set to 32 on SPU, and may be set to 32 as well
on PowerPC.  Since the new gdbarch_significant_addr_bit defaults to the
value of gdbarch_addr_bit, this means addresses may be improperly truncated.

Work around this problem by explicitly setting gdbarch_significant_addr_bit
to 64 both for the SPU target and also for PowerPC target that support
Cell/B.E. execution.

gdb/ChangeLog:
2017-12-20  Ulrich Weigand  <uweigand@de.ibm.com>

	* spu-tdep.c (spu_gdbarch_init): Set set_gdbarch_significant_addr_bit
	to 64 bits.
	(ppc_linux_init_abi): Likewise, if Cell/B.E. is supported.