target/i386: do not block singlestep for STI

target/i386: do not block singlestep for STI

STI will trigger a singlestep exception even if it has inhibit-IRQ
behavior.  Do not suppress single-step for all IRQ-inhibiting
instructions, instead special case MOV SS and POP SS.

Cc: qemu-stable@nongnu.org
Fixes: f0f0136abba ("target/i386: no single-step exception after MOV or POP SS", 2024-05-25)
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
(cherry picked from commit 1e94ddc6854431064c94a7d8f2f2886def285829)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>