Integrate perlio:

[  8457]
Provide infrastructure for PERL_ASYNC_CHECK() style safe signals.
Provides all the "cost" but no benefit yet - it is to allow cost
to be measured, and implementation experiments (just in mg.c?).

p4raw-link: @8457 on //depot/perlio: ce08f86c8b1d404b3d9fec75a102b8cd65f8766a

p4raw-id: //depot/perl@8458