amd64: initialize td_frame stack area for init(8) main thread

Unitialized td_frame mostly does not matter since all registers are
overwritten on exec to activate init(8). Except PSL_T bit from the
%rflags which might leak into fresh init as garbage, causing spurious
SIGTRAPs delivered to init until first syscall is executed.

Reviewed by: emaste, jhb, jhibbits
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D44498