Currently, we are hardcoding the ISA string in the device tree
generator. The ISA string from the device tree affects which
ISA extensions will be used by the bootloader/kernel.
This function allows generating the ISA string from the gem5's
ISA object rather than using hardcoded values.
This series of changes also correct a couple of hardcoded
RISC-V ISA strings in the standard library, as well as not
enable RVV instructions for the U74 core model.
Signed-off-by: Hoa Nguyen <hn@hnpl.org>