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 <firstname.lastname@example.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.