arm64/sve: ptrace and ELF coredump support

This patch defines and implements a new regset NT_ARM_SVE, which
describes a thread's SVE register state.  This allows a debugger to
manipulate the SVE state, as well as being included in ELF
coredumps for post-mortem debugging.

Because the regset size and layout are dependent on the thread's
current vector length, it is not possible to define a C struct to
describe the regset contents as is done for existing regsets.
Instead, and for the same reasons, NT_ARM_SVE is based on the
freeform variable-layout approach used for the SVE signal frame.

Additionally, to reduce debug overhead when debugging threads that
might or might not have live SVE register state, NT_ARM_SVE may be
presented in one of two different formats: the old struct
user_fpsimd_state format is embedded for describing the state of a
thread with no live SVE state, whereas a new variable-layout
structure is embedded for describing live SVE state.  This avoids a
debugger needing to poll NT_PRFPREG in addition to NT_ARM_SVE, and
allows existing userspace code to handle the non-SVE case without
too much modification.

For this to work, NT_ARM_SVE is defined with a fixed-format header
of type struct user_sve_header, which the recipient can use to
figure out the content, size and layout of the reset of the regset.
Accessor macros are defined to allow the vector-length-dependent
parts of the regset to be manipulated.

Signed-off-by: Alan Hayward <alan.hayward@arm.com>
Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Cc: Alex Bennée <alex.bennee@linaro.org>
Cc: "Okamoto, Takayuki" <tokamoto@jp.fujitsu.com>

---

**Dropped at v3** Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
due to non-trivial bug-fixes and changes requested by reviewers
after v2.

Changes since v3
----------------

Bug fixes:

 * __fpr_get(): don't snapshot the targets fpsimd regs until after
   they have been updated by sve_sync_to_fpsimd().

   Without this, a short regset write may cause old register values
   to mysteriously reappear.

Requested by Catalin Marinas:

 * sve_set(): Remove duplicate call to sve_sync_to_fpsimd(): __fpr_set()
   already does it.

 * Replace static __maybe_unused functions with static inlines.

Miscellaneous:

 * Add comments explaining the non-obvious purpose of the _sync_to_
   calls in __fpr_set()/sve_set().