tests/sys/kern: stress FP registers across sigreturn on powerpc64HEADmain

This is the program that found the races fixed in 1f1043d550ea. One
thread keeps a pattern in f14-f31 across a signal, while helpers pinned
to the same CPU keep different patterns in the real registers and get
woken up from another CPU to preempt it at random points. If the kernel
ever hands it somebody else's registers, the pattern doesn't match.

The register loads, the thr_kill() and the handshake with the helper are
in assembly so that nothing touches the FP registers between sigreturn
and the check. It runs for ten seconds and is skipped with fewer than
two CPUs.

Requested by:	adrian
Approved by:	jhibbits
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D60411

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