x86: Initialize segment selectors

Currently, the BSP's segment selectors are not initialized in 32-bit
(cstart.S). As a result the tests implicitly rely on the segment
selector values that are set by the BIOS. If this assumption is not
kept, the task-switch test fails.

Fix it by initializing them.

Signed-off-by: Nadav Amit <namit@vmware.com>
Message-Id: <20200623084132.36213-1-namit@vmware.com>
Reviewed-by: Jim Mattson <jmattson@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>