FPU init: allocate initial state from UMA to ensure alignment

The Intel Instruction Set Reference says this about the XSAVE instruction:

Use of a destination operand not aligned to 64-byte boundary
(in either 64-bit or 32-bit modes) results in a general-protection
(#GP) exception.

This alignment happens naturally when all malloc buckets are powers
of two. However, this change is necessary on some systems when
certain non-power-of-two (and non-multiple of 64) malloc buckets
are defined.

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