Backport support for fs_base/gs_base and $_siginfo from GDB master.

[jhb]

Summary

Bring in two patchsets committed to GDB master along with a few other commits
those patchsets depend on.

• Add support for examining $_siginfo. When a thread has valid signal information, report the current information in a $_siginfo variable. For live processes the values are extracted via the PT_LWPINFO ptrace request. The values are also extracted from process core dumps that include NT_LWPINFO notes (currently only present in FreeBSD 12-current).
• Add support for the $fs_base and $gs_base registers on amd64. This only works for live processes via the PT_{GET,SET}{FS,GS}BASE ptrace requests. FreeBSD currently does not store these values in core dumps, so they are not available from core dumps.

Test Plan

• Tested new features on FreeBSD/amd64 head.
• poudriere testport on 10.3/amd64, 10.3/i386, 11.1/amd64, 12.0/amd64.

[luca.pizzamiglio_gmail.com]

It looks good to me. My build and run tests are positive.

[imp]

This looks good to me, assuming the patches are good. The ports part looks fine, and listing the patches in order I think will insulate from ordering issues that would otherwise be present.