RISC-V: Name newly added flags in changelog

This patch fixes the changelog to explicitly name the added command line
flags introduced in this patch:
https://gcc.gnu.org/pipermail/gcc-patches/2023-April/616807.html



2023-05-01 Patrick O'Neill <patrick@rivosinc.com>

Signed-off-by: Patrick O'Neill <patrick@rivosinc.com>