Update GDB port to 7.9.

[eadler]

Summary

Update to 7.9.

• Move changes that have been adopted upstream after 7.9 into extra patches named after the commit to GDB's master branch. This includes previous patches such as the signal trampoline detection fixes and XSAVE support as well as well as new fixes such as fixing 'gcore' to not require procfs.
• The tui-io.c patch previously needed with new readline has been accepted upstream.
• The remaining patches have been updated to apply cleanly.
• The threads target has been fixed to actually pass register requests for LWP-backed threads down to the ptrace backends without detouring via libthread_db which actually fixes AVX with threads.

Test Plan

• Port builds on 10.0/amd64, 10.0/i386, and HEAD/amd64. Run tested on HEAD/amd64.
• A git branch matching the port was also tested on HEAD/i386.

[eadler]

take as mine