tests/drivers/ipm: Enable on (uniprocessor) x86_64

This test was having trouble earlier (at least with some toolchains),
but whatever was causing that seems to have been fixed.  The only
remaining issue is that neither the IPM console code nor the test are
SMP-safe.  There's a bug (#14639) tracking the need to get these
working, but for now the straightforward workaround is just to disable
SMP.

And even long term, IPM is an oddball interprocessor communication
mechanism designed for asymmetric multiprocessing devices like Quark
SE and doesn't seem like an obvious fit for a SMP machine.

Fixes #12478

Signed-off-by: Andy Ross <[email protected]>

Author: Andy Ross

tests/drivers/ipm/prj.conf

@@ -7,3 +7,5 @@ CONFIG_IPM_CONSOLE_SENDER=y
 CONFIG_IRQ_OFFLOAD=y
 CONFIG_MAIN_STACK_SIZE=1024
 CONFIG_COVERAGE=n
+
+CONFIG_SMP=n

tests/drivers/ipm/testcase.yaml

@@ -2,5 +2,4 @@ tests:
   peripheral.mailbox:
     filter: not CONFIG_SOC_QUARK_SE_C1000_SS
     arch_exclude: posix xtensa
-    platform_exclude: qemu_x86_64 # see issue #12478
     tags: drivers ipc