target-i386: Move user-mode exception actions out of user-exec.c

The exception_action() function in user-exec.c is just a call to
cpu_loop_exit() for every target CPU except i386. Since this
function is only called if the target's handle_mmu_fault() hook has
indicated an MMU fault, and that hook is only called from the
handle_cpu_signal() code path, we can simply move the x86-specific
setup into that hook, which allows us to remove the TARGET_I386
ifdef from user-exec.c.

Of the actions that were done by the call to raise_interrupt_err():
* cpu_svm_check_intercept_param() is a no-op in user mode
* check_exception() is a no-op since double faults are impossible
for user-mode
* assignments to cs->exception_index and env->error_code are no-ops
* assigning to env->exception_next_eip is unnecessary because it
is not used unless env->exception_is_int is true
* cpu_loop_exit_restore() is equivalent to cpu_loop_exit() since
pc is 0
which leaves just setting env_>exception_is_int as the action that
needs to be added to x86_cpu_handle_mmu_fault().

Backports commit 0c33682d5f29b0a4ae53bdec4c8e52e4fae37b34 from qemu

qemu/target-i386/helper.c

@@ -514,6 +514,8 @@ int x86_cpu_handle_mmu_fault(CPUState *cs, vaddr addr,
     env->error_code = (is_write << PG_ERROR_W_BIT);
     env->error_code |= PG_ERROR_U_MASK;
     cs->exception_index = EXCP0E_PAGE;
+    env->exception_is_int = 0;
+    env->exception_next_eip = -1;
     return 1;
 }