Commit 71560d1 ("[librm] Preserve FPU, MMX and SSE state across calls
to virt_call()") added FXSAVE and FXRSTOR instructions to iPXE.  In
KVM virtual machines, these instructions execute fine as long as the
host CPU supports the "unrestricted_guest" feature (that is, it can
virtualize big real mode natively).  On older host CPUs however, KVM
has to emulate big real mode, and it currently doesn't implement
FXSAVE emulation.

Upstream QEMU rebuilt iPXE at commit 0418631 ("[thunderx] Fix
compilation with older versions of gcc") which is a descendant of
commit 71560d1 (see above).

This was done in QEMU commit ffdc5a2 ("ipxe: update submodule from
4e03af8ec to 041863191").  The resultant binaries were bundled with
the QEMU v2.7.0 release; see QEMU commit c52125a ("ipxe: update
prebuilt binaries").

This distributed the iPXE workaround for the Tivoli VMM bug to a
number of KVM users with old host CPUs, causing KVM emulation failures
(guest crashes) for them while netbooting.

Make the FXSAVE and FXRSTOR instructions conditional on a new feature
test macro called TIVOLI_VMM_WORKAROUND.  Define the macro by default.

There is prior art for an assembly file including config/general.h:
see arch/x86/prefix/romprefix.S.  Also, TIVOLI_VMM_WORKAROUND seems to
be a good fit for the "Obscure configuration options" section in
config/general.h.

Cc: Bandan Das <bsd@redhat.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Greg <rollenwiese@yahoo.com>
Cc: Michael Brown <mcb30@ipxe.org>
Cc: Michael Prokop <launchpad@michael-prokop.at>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Peter Pickford <arch@netremedies.ca>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Ref: https://bugs.archlinux.org/task/50778
Ref: https://bugs.launchpad.net/qemu/+bug/1623276
Ref: https://bugzilla.proxmox.com/show_bug.cgi?id=1182
Ref: https://bugzilla.redhat.com/show_bug.cgi?id=1356762
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>