kselftest/arm64: Test that ptrace takes effect in the target process

While we have test coverage for the ptrace interface in our selftests
the current programs have a number of gaps. The testing is done per
regset so does not cover interactions and at no point do any of the
tests actually run the traced processes meaning that there is no
validation that anything we read or write corresponds to register values
the process actually sees. Let's add a new program which attempts to cover
these gaps.

Each test we do performs a single ptrace write. For each test we generate
some random initial register data in memory and then fork() and trace a
child. The child will load the generated data into the registers then
trigger a breakpoint. The parent waits for the breakpoint then reads the
entire child register state via ptrace, verifying that the values expected
were actually loaded by the child. It then does the write being tested
and resumes the child. Once resumed the child saves the register state
it sees to memory and executes another breakpoint. The parent uses
process_vm_readv() to get these values from the child and verifies that
the values were as expected before cleaning up the child.

We generate configurations with combinations of vector lengths and SVCR
values and then try every ptrace write which will implement the
transition we generated. In order to control execution time (especially
in emulation) we only cover the minimum and maximum VL for each of SVE
and SME, this will ensure we generate both increasing and decreasing
changes in vector length. In order to provide a baseline test we also
check the case where we resume the child without doing a ptrace write.

In order to simplify the generation of the test count for kselftest we
will report but skip a substantial number of tests that can't actually
be expressed via a single ptrace write, several times more than we
actually run. This is noisy and will add some overhead but is very much
simpler so is probably worth the tradeoff.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20240122-arm64-test-ptrace-regs-v1-1-0897f822d73e@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>