Support all of the easy GM block sizes.
Use direct memory operations, since the pointers are aligned.

While BS=2 (16 bytes, 1 tag) is a legal setting, that requires
an atomic store of one nibble.  This is not difficult, but there
is also no point in supporting it until required.

Note that cortex-a710 sets GM blocksize to match its cacheline
size of 64 bytes.  I expect many implementations will also
match the cacheline, which makes 16 bytes very unlikely.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20230811214031.171020-4-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>