- boostorg#255

This implements the required asm functions for sparc64_sysv_elf and
was tested on an OpenBSD/sparc64 system.

Jump_fcontext uses an extra C call frame to store the frame-pointer and
return address. Because of this the code is simply a save (to new reg
window), then forcing a window flush and finally switch stack and restore
from there.

Since jump_fcontext() uses a register window and stack frame, make_fcontext()
reserves two call frames on the stack (one for jump_fcontext() and the other
for the callback function).

OpenBSD/sparc64 uses stackghost which prevents userland from overriding the
return-address on the stack. Because of this make_fcontext() uses an extra
trampoline to implement the _exit(0) call if the callback returns.

All tests pass with this on OpenBSD/sparc64 (also the tests for fiber,
coroutine and coroutine2).

Without this test_fcontext.cpp fails on OpenBSD with a not-on-stack OS error.

…a source in install targets.

…targets.