FPU init: allocate initial state from UMA to ensure alignment

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