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[lldb][docs] Add documentation for AArch64 Linux SME only support (#165415)

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As for other features, this is a mixture of user facing decisions and
internal design decisions.
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`zt0`'s value and whether it is active or not will be saved prior to
expression evaluation and restored afterwards.
## SME Only Systems
AArch64 systems may have both SVE and SME, or they can have only SME. If they
only have SME, the system has the usual SVE state, but that state and SVE
instructions may only be used while in streaming mode.
The LLDB experience is very similar to SVE+SME systems, with a few changes.
### Registers
When in streaming mode, registers act as they would on an SVE+SME system.
Outside of streaming mode LLDB will show the `Z` registers as zero extended copies
of the `V` registers.
Writes to `Z` registers are allowed, but these are converted to `V` register writes
and so only the bottom 128-bits will be applied. The size of the value
written to a `Z` register must match the current streaming vector length, even if
the process is in non-streaming mode.
For an SME only system, `lldb-server` describes the system as having `Z`
registers with `V` registers as subsets of those registers. We do not change the
representation each time the mode changes.
A consequence of this is that if the user writes to a `V` register while in
non-streaming mode, it will be sent to `lldb-server` as a `Z` register write
of a zero extended value. `lldb-server` will convert that back into a `V`
register write with the value truncated to 128-bits.
In streaming mode, it will be zero extended, sent as a `Z` write and written
to the real streaming SVE `Z` register without truncation.
`P` registers will be shown as 0s in non-streaming mode. They cannot be
written to in non-streaming mode.
The `ffr` register will also be shown as 0s in non-streaming mode. In streaming
mode, use of `ffr` is forbidden so it will also show as 0s. It cannot be written
to in either state.
(in the former state, LLDB generates the fake value, in the latter state, the
kernel tells `lldb-server` that the value is 0, the result is the same)
The `ZA` and `ZT0` registers act as they would for an SVE+SME system.
Since there is no non-streaming SVE, there is no non-streaming vector length.
Therefore even in non-streaming mode, the value shown in `vg` will be the
streaming vector length, and it will be equal to the value of `svg`.
### Expression Evaluation
Some instructions are illegal to use in streaming mode, unless `FEAT_SMEFA64`
is present.
LLDB **will not** make any attempt to make expressions compatible with the
current mode.
If part of an expression is not compatible, it will
result in a `SIGILL` that will be cleaned up as any other signal would be. All
register, `ZA` and mode state will be restored as normal after expression
evaluation.
Note that to restore to a non-streaming state from a streaming state, LLDB uses
a special part of the Linux Kernel's
[SVE ABI](https://docs.kernel.org/arch/arm64/sve.html). FPSIMD format data is
written to the non-existent non-streaming SVE register set, with the vector
length set to 0 to cause the process to exit streaming mode and apply those
FPSIMD values to the `V` registers.
This is only used for this purpose. Otherwise, in non-streaming mode FP
registers are accessed using the FP register set, and in streaming mode using
the streaming SVE register set.
## Guarded Control Stack Extension (GCS)
GCS support includes the following new registers: