x86: assorted IAMCU CPU checking fixes

x86: assorted IAMCU CPU checking fixes

The checks done by check_cpu_arch_compatible() were halfway sensible
only at the time where only L1OM support was there. The purpose,
however, has always been to prevent bad uses of .arch (turning off the
base CPU "feature" flag) while at the same time permitting extensions to
be enabled / disabled. In order to achieve this (and to prevent
regressions when L1OM and K1OM support are removed)
- set CpuIAMCU in CPU_IAMCU_FLAGS,
- adjust the IAMCU check in the function itself (the other two similarly
  broken checks aren't adjusted as they're slated to be removed anyway),
- avoid calling the function for extentions (which would never have the
  base "feature" flag set),
- add a new testcase actually exercising ".arch iamcu" (which would also
  regress with the planned removal).