MFC Loader Interpreter Coexistance: r337806, r337868, r337914, r337927,
r337989, r338052, r338064, r338407, r338418, r338474

r337806:
Create a loader for each interpreter for x86 BIOS and all EFI

Create loader_{4th,lua,simp}{,.efi}. All of these are installed by
default. Create LOADER_DEFAULT_INTERP to specify the default
interpreter when no other is specified. LOADER_INTERP is the current
interpreter language building. Turn building of lua on by default to
match 4th. simploader is a simplified loader build w/o any interpreter
language (but with a simple loader). This is the historic behavir you
got with WITHOUT_FORTH. Make a hard link to the default loader. This
has to be a hard link rather than the more desirable soft link because
older zfsboot blocks don't support symlinks.

r337868:
stand: Use -Oz/-Os for all loader/stand builds.

While we're not super size constrained, the x86 BIOS /boot/loader has
to be less than about 520k-530k to be reliable. The LUA loader is at
this size today. -Oz saves 15-20% on the size, keeping us safely small
enough (comparable to where we were with the 4th loader). This will
also help with sjg's work on bringing in bearssl, though we may again
be looking for space in the LUA loader.

Size table for clang 6.0.0:
default -O1 -Os -Oz
4th 442368 417792 389120 376832
lua 524288 479232 446464 430080

r337914:
Install links for loader.efi.

r337927:
Add ashldi3 and ashrdi3 to mips.

Now that we're using -Os, mips needs these routines.

r337989:
Copy the boot loader from the new location for the co-existing
loaders.

r338052:
libsa: Add lshrdi3.c for powerpc* and mips

r338064:
Turn back the clock just a little: make userboot.so always be 4th

Turns out there was a hidden dependency we hasn't counted upon. The
host load /boot/userboot.so to boot the VMs it runs. This means that
the change to lua meant suddently that nobody could run their older
VMs because LUA wasn't in 10.0, last month's HardenedBSD, 11.2 or
whatever. Even more than for the /boot/loader* binaries, we need a
good coexistance strategy for this. While that's being designed and
implemented, drop back to always 4th for userboot.so. This will fail
safe in all but the most extreme environments (but lua-only hacks
to .lua files won't be processes in VMs until we fix it).

r338407:
lualoader: Print error messages from command failures at the prompt

Previously lualoader would remain silent, rather than printing
command_errmsg or noting that a command had failed or was not found.

r338418:
userboot: handle guest interpreter mismatches more intelligently

The switch to lualoader creates a problem with userboot: the host is
inclined to build userboot with Lua, but the host userboot's interpreter
must match what's available on the guest. For almost all FreeBSD guests in
the wild, Lua is not yet available and a Lua-based userboot will fail.

This revision updates userboot protocol to version 5, which adds a
swap_interpreter callback to request a different interpreter, and tries to
determine the proper interpreter to be used based on how the guest
/boot/loader is compiled. This is still a bit of a guess, but it's likely
the best possible guess we can make in order to get it right. The
interpreter is now embedded in the resulting executable, so we can open
/boot/loader on the guest and hunt that down to derive the interpreter it
was built with.

Using -l with bhyveload will not allow an intepreter swap, even if the
loader specified happens to be a userboot with the wrong interpreter. We'll
simply complain about the mismatch and bail out.

For legacy guests without the interpreter marker, we assume they're 4th.
For new guests with the interpreter marker, we'll read it and swap over
to the proper interpreter if it doesn't match what the userboot we're using
was compiled with.

Both flavors of userboot are installed by default, userboot_4th.so and
userboot_lua.so. This fixes the build WITHOUT_FORTH as a coincidence, which
was broken by userboot being forced to 4th.

r338474:
Be a little conservative about when to force size optimizations.

Reports have come in that there's issue with powerpc and sparc64 since
we've switched to using -Oz / -Os. We don't strictly need them for
!x86, so be conservative about when we enable them.