target/i386: Check CR0.TS before enter_mmx

target/i386: Check CR0.TS before enter_mmx

When CR0.TS=1, execution of x87 FPU, MMX, and some SSE instructions will
cause a Device Not Available (DNA) exception (#NM). System software uses
this exception event to lazily context switch FPU state.

Before this patch, enter_mmx helpers may be generated just before #NM
generation, prematurely resetting FPU state before the guest has a
chance to save it.

Signed-off-by: Matt Borgerson <contact@mborgerson.com>
Message-ID: <CADc=-s5F10muEhLs4f3mxqsEPAHWj0XFfOC2sfFMVHrk9fcpMg@mail.gmail.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>