test: x86: test SIMD exception delivery

Fails on all test points, of course.
receive #XM via IDT, report it as GP for INT19

Makes the test case pass. Up next: same for ye olde x87.
test: x86: add test for delivery of x87 faults

The test expects these to be reported as though #GP for int #16. This may
deviate from how L4Ka::Pistachio does it.
exception: report x86 #MF to userspace

The receiving exception handler may point the faulting thread (using
ExchangeRegisters) to a code fragment that captures the FPU status word and
acts accordingly, later popping an exception that lets the handler cause its
return to the x87 instruction that originally faulted.
exception: make debug prints more useful

Removes the #XM indicator, and only (always) dumps #GP info for userspace when
return_from_gp() cannot find an exception handler.