Introduce two common helpers to take care of splitting and recombining
YMM registers to/from XSAVE-like data. Since FreeBSD, Linux and NetBSD
all use XSAVE-like data structures but with potentially different field
layouts, the function takes two pointers -- to XMM register and to YMM
high bits, and copies the data from/to YMMReg type.
While at it, remove support for big endian. To mine and Pavel Labath's
combined knowledge, there is no such thing on x86. Furthermore,
assuming that the YMM register data would be swapped for big endian
seems to be a weird assumption.
// NB: I have no clue why FreeBSD code claims to belong in 'POSIX', and Linux does not.