riscv: Enable Vector code to be built

This patch adds configs for building Vector code. First it detects the
reqired toolchain support for building the code. Then it provides an
option setting whether Vector is implicitly enabled to userspace.

Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
Co-developed-by: Greentime Hu <greentime.hu@sifive.com>
Signed-off-by: Greentime Hu <greentime.hu@sifive.com>
Suggested-by: Conor Dooley <conor.dooley@microchip.com&gt;>
Co-developed-by: Andy Chiu <andy.chiu@sifive.com>
Signed-off-by: Andy Chiu <andy.chiu@sifive.com>