Unset gdbarch significant_addr_bit by default

This patch fixes a bug introduced by fix to AArch64 pointer tagging.

In our fix for tagged pointer support our agreed approach was to sign
extend user-space address after clearing tag bits. This is not same
for all architectures and this patch allows sign extension for
addresses on targets which specifically set significant_addr_bit.

More information about patch that caused the issues and discussion
around tagged pointer support can be found in links below:

https://sourceware.org/ml/gdb-patches/2018-05/msg00000.html
https://sourceware.org/ml/gdb-patches/2017-12/msg00159.html

gdb/ChangeLog:

2018-05-31  Omair Javaid  <omair.javaid@linaro.org>

	PR gdb/23210
	* gdbarch.sh (significant_addr_bit): Default to zero when
	not set by target architecture.
	* gdbarch.c: Re-generated.
	* utils.c (address_significant): Update.