FPU init: Do potentially blocking operations before disabling interrupts

In particular, uma_zcreate creates sysctl oids, which locks an sx lock,
which uses IPIs under contention. IPIs tend not to work very well
when interrupts are disabled. Who knew, right?

Reviewed by: cem kib
MFC after: 2 weeks
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D25098