Add fork tracing.

[jhb]

Summary

• Apply upstream patch to add fork tracing support using PT_FOLLOW_FORK. This allows child and/or parent processes to be followed after a fork and also includes support for catching fork/vfork events via 'catch'.
• Apply upstream patch to add exec tracing support which allows exec events to be caught.
• Enable support for 64-bit targets on 32-bit systems (e.g. allow the i386 gdb to debug an amd64 binary).

Test Plan

• Tested poudriere builds of 10.0 i386 and amd64 and recent HEAD amd64.

[jhb]

Backport fix from GDB mainline to remove Linuxism in MI fork following code.

[jhb]

Included in update to 7.10.