KVM: selftests: disable stack protector for all KVM tests

Since 4.8.3, gcc has enabled -fstack-protector by default.  This is
problematic for the KVM selftests as they do not configure fs or gs
segments (the stack canary is pulled from fs:0x28).  With the default
behavior, gcc will insert a stack canary on any function that creates
buffers of 8 bytes or more.  As a result, ucall() will hit a triple
fault shutdown due to reading a bad fs segment when inserting its
stack canary, i.e. every test fails with an unexpected SHUTDOWN.

Fixes: 14c47b75 ("kvm: selftests: introduce ucall")
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>