kunit: tool: show list of valid --arch options when invalid

Consider this attempt to run KUnit in QEMU:
$ ./tools/testing/kunit/kunit.py run --arch=x86

Before you'd get this error message:
kunit_kernel.ConfigError: x86 is not a valid arch

After:
kunit_kernel.ConfigError: x86 is not a valid arch, options are ['alpha', 'arm', 'arm64', 'i386', 'powerpc', 'riscv', 's390', 'sparc', 'x86_64']

This should make it a bit easier for people to notice when they make
typos, etc. Currently, one would have to dive into the python code to
figure out what the valid set is.

Signed-off-by: Daniel Latypov <dlatypov@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>