Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

/* -> /*- for copyright notices, minor format tweaks as necessary

Remove __aligned(16) from the definition of struct _ia64_fpreg. It's
a non-standard construct. Instead, redefine struct _ia64_fpreg as a
union and put a long double in it. On ia64 and for LP64, this is
defined by the ABI to have 16-byte alignment. For ILP32 a long double
has 4-byte alignment, but we don't support ILP32.

Note that the in-memory image of a long double does not match the in-
memory image of spilled FP registers. This means that one cannot use
the fpr_flt field to interpet the bits. For this reason we continue
to use an aggregate type.

We sloppily created an array for the high FP registers (f32-f127),
but this just created a weird inconsistency when porting gdb(1).
Instead, we name each high FP register seperately, like we do for
all the other registers.

Make the regset pointers const pointers for the context restore functions.
This works better with set_mcontext() and is more precise in general.

This file creates register sets based on the runtime specification.
The advantage of using register sets is that you don't focus on each
register seperately, but instead instroduce a level of abstraction.
This reduces the chance of errors, and also simplifies the code.
The register sers form the basis of everything register.
The sets in this file are:

struct _special
contains all of the control related registers, such as instruction
pointer and stack pointer. It also contains interrupt specific registers
like the faulting address. The set is roughly split in 3 groups. The
first contains the registers that define a context or thread. This is
the only group that the kernel needs to switch threads. The second group
contains registers needed in addition to the first group needed to switch
userland threads. This group contains the thread pointer and the FP control
register. The third group contains those registers we need for execption
handling and are used on top of the first two groups.

struct _callee_saved, struct _callee_saved_fp
These sets contain the preserved registers, including the NaT after
spilling. The general registers (including branch registers) are
seperated from the FP registers for ptrace(2).

struct _caller_saved, struct _caller_saved_fp
These sets contain the scratch registers based on SDM 2.1, This means that
both ar.csd and ar.ccd are included here, even though they contain ia32
segment register descriptions. We keep seperate NaT bits for scratch and
preserved registers, because they are never saved/restored at the same
time.

struct _high_fp
The upper 96 FP registers that can be enabled/disabled seperately on
the CPU from the lower 32 FP registers. Due to the size of this set,
we treat them specially, even though they are defined as scratch
registers.

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