target/riscv/cpu: add misa_ext_info_arr[]

target/riscv/cpu: add misa_ext_info_arr[]

Next patch will add KVM specific user properties for both MISA and
multi-letter extensions. For MISA extensions we want to make use of what
is already available in misa_ext_cfgs[] to avoid code repetition.

misa_ext_info_arr[] array will hold name and description for each MISA
extension that misa_ext_cfgs[] is declaring. We'll then use this new
array in KVM code to avoid duplicating strings. Two getters were added
to allow KVM to retrieve the 'name' and 'description' for each MISA
property.

There's nothing holding us back from doing the same with multi-letter
extensions. For now doing just with MISA extensions is enough.

It is worth documenting that even using the __bultin_ctz() directive to
populate the misa_ext_info_arr[] we are forced to assign 'name' and
'description' during runtime in riscv_cpu_add_misa_properties(). The
reason is that some Gitlab runners ('clang-user' and 'tsan-build') will
throw errors like this if we fetch 'name' and 'description' from the
array in the MISA_CFG() macro:

../target/riscv/cpu.c:1624:5: error: initializer element is not a
                              compile-time constant
    MISA_CFG(RVA, true),
    ^~~~~~~~~~~~~~~~~~~
../target/riscv/cpu.c:1619:53: note: expanded from macro 'MISA_CFG'
    {.name = misa_ext_info_arr[MISA_INFO_IDX(_bit)].name, \
             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~

gcc and others compilers/builders were fine with that change. We can't
ignore failures in the Gitlab pipeline though, so code was changed to
make every runner happy.

As a side effect, misa_ext_cfg[] is no longer a 'const' array because
it must be set during runtime.

Suggested-by: Andrew Jones <ajones@ventanamicro.com>
Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20230706101738.460804-12-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>