powerpc64: Add a trap stack area

Currently, the trap code switches to the the temporary stack in the dbtrap
section. It works in most cases, but in the beginning of the execution, the
temp stack is being used, as starting in the powerpc_init() code.

In this current scenario, the stack is being overwritten, which causes the
return of breakpoint() to take abnormal execution.

This current patchset create a small stack to use by the dbtrap: codepath
avoiding the corruption of the temporary stack.

PR: 224872
Submitted by: breno.leitao_gmail.com
Reviewed by: jhibbits
Differential Revision: https://reviews.freebsd.org/D14484