Copy stable/10@r272459 to releng/10.1 as part of
the 10.1-RELEASE process.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Rename the libc signal trampoline to __sigtramp to match netbsd. This
should allow gdb to detect when we're executing in a signal trampoline.

Add an alternate signal trampoline to libc; add a wrapper for the sigtramp
install sysarch, to be called from _start.

This will allow the stack to be mapped non-executable, as required by the
sparc v9 abi.