With gcc version >= 11.1.0, to support new ISA spec changes, which
moved some instructions from the I extension to the Zicsr and Zifencei
extensions, we have to explicitly specify Zicsr and Zifencei via -march.
But it is not required for old gcc versions.
To cope with both cases, we use rv32g instead of rv32ima.
"g" = "imafd". RVOS doesn't use "f" & "d", and we also don't want "c".
We use "g" to just to make life easy, otherwise we may have to intriduce
some mechanism to judge and differ the version of gcc used.
Also updated some comments to move to rv32g and don't involve words such
as "rv32ima".
Signed-off-by: Chen Wang <wangchen20@iscas.ac.cn>
