sme: Add SVE/SME testcases

sme: Add SVE/SME testcases

Add 5 SVE/SME tests to exercise all the new features like reading/writing
registers, pseudo-registers, signal frames and core files.

- Sanity check for SME: Gives a brief smoke test to make sure the most basic
of features are working correctly.

- ZA unavailability tests: Validates the behavior/content of the ZA register
is correct when no payload is available.  It also exercises changing the
vector lengths.

- ZA availability tests: These tests exercise reading/writing to all the
possible ZA pseudo-registers, and validates the state is correct.

- Core file tests: Validates that core file reading and writing works
correctly and that all state dumped/loaded is sane.  This is exercised for
both Linux Kernel core files and gcore core files.

- Signal frame tests: Validates the correct restoration of SME/SVE/FPSIMD
values across signal frames.

Since some of these tests are very lengthy and take a little while to run
(under QEMU at the moment), I decided to parallelize them into smaller
chunks so we can throw some more CPU power at them so they run faster.

I'd still like to add a few more tests to give the testsuite more coverage
in the areas of SME/SVE.  Hopefully in the near future that will happen.

Just a reminder that these SME tests are currently unsupported when gdb is
connected to a remote target.  That's because the RSP doesn't support
communicating changes in vector lenghts mid-execution, so gdb will always
get wrong state from the remote target.

Co-Authored-By: Ezra Sitorus <ezra.sitorus@arm.com>
Reviewed-by: Thiago Jung Bauermann <thiago.bauermann@linaro.org>