Add x86 PT_GETFSBASE, PT_GETGSBASE machine-depended ptrace requests to
obtain the thread %fs and %gs bases. Add x86 PT_SETFSBASE and
PT_SETGSBASE requests to set the bases from debuggers. The set
requests, similarly to the sysarch({I386,AMD64}_SET_FSBASE),
override the corresponding segment registers.

The main purpose of the operations is to retrieve and modify the tcb
address for debuggee.

Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks