[lldb][AArch64][Linux] Add tests for SME only support

This PR is the tests for #138717. I have split it from implementation
as there is a lot of code, and the two don't really overlap. The tests
are checking what a user will see in LLDB, and only indirectly what
happens in lldb-server.

There are two types of tests, the first is register access tests.
These start in a given mode, where a mode is a combination of the
streaming mode on or off, ZA on or off, and a vector length.

For register access tests I have used two states:
* Streaming mode, ZA on, default vector length
* Non-streaming mode, ZA off, default vector length

(changing mode and vector length is tested in existing SVE+SME tests
and the expression tests I'm adding in this same PR)

The test program:
* Sets up the requested mode.
* Allocates a bunch of buffers for LLDB to write expected register
  values to.
* Sets initial register values.

Then LLDB looks for those expected values. Assuming they are present,
it then writes a new set of values to the registers, and to the buffers
within the program. The program is then stepped, which runs an in-process
function to check that it now sees those register values. If that function
fails, we have a bug. If it does not, LLDB then checks for the same values.

This process repeats for several registers. Hence the repeated calls
to check_register_values in the test program.

In this way we make sure that LLDB sends the correct values to ptrace
by validating them in process. Also that LLDB does not write other registers
in the process of writing one register. This happened to me several times
during development.

The buffer writes are done with memory writes not expression evaluation
becuase at this stage we cannot trust that expressions work
(the expression tests will later prove that they do).

The second type of tests are epxression state restoration tests.
For each combination of states we check that we can restore to a given
state if an expression takes us into the other state.

This includes between the same state, and adds an extra vector lenth
to check expanding and shrinking SVE buffers. This produces roughly 64
tests.

I have written them all as one test case in a loop because these take
a long time (~20 minutes on an FVP) and you almost always want it to fail
fast. When tracing is enabled you'll see the current states logged.

It's possible that we could skip some of these tests as being very unlikely
to ever happen, but in the spirit of
"There are Only Four Billion Floats–So Test Them All!" I think it is simpler
to enumerate every possible state change.

I have not added a test for executing an expression incompatible with
the current mode because this leads to SIGILL, which I know we already
handle.

At this time LLDB makes no effort to make the expression compatible
with the current mode either.

Author: DavidSpickett

lldb/test/API/linux/aarch64/sme_only_registers/Makefile

@@ -0,0 +1,6 @@
+C_SOURCES := main.c
+# The memset provided by glibc may use instructions we cannot use in
+# streaming mode.
+CFLAGS_EXTRAS := -march=armv8-a+sve+sme+sme2 -fno-builtin-memset
+
+include Makefile.rules