MFC r266778:

MFC r266778:
Repair nested signal handling on PowerPC. The signal trampoline code
was not allocating space for the parameter save area in the stack frame.
If the compiler chose to save the argument to the signal handler on the
stack, it would overwrite the first 32 bits of the sigaction struct with
it, corrupting it for a subsequent invocation.

PR:		powerpc/183040

git-svn-id: svn://svn.freebsd.org/base/stable/10@267040 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f