newlib: try to fix newlib performance against coremark/dhrystone

See merge request software/riscv-gnu-toolchain/riscv-gnu-toolchain!1

Signed-off-by: Huaqi Fang <578567190@qq.com>

This patch is extracted from 60515b36
and 89e2c05d

This patch adds Nuclei's p-extension support.

* riscv-gcc: accepts p-extension flags.
* riscv-binutils: supports emitting p-extension instructions.