riscv: Introduce Vector enable/disable helpers

These are small and likely to be frequently called so implement as
inline routines (vs. function call).

Co-developed-by: Guo Ren <guoren@linux.alibaba.com>
Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
Co-developed-by: Vincent Chen <vincent.chen@sifive.com>
Signed-off-by: Vincent Chen <vincent.chen@sifive.com>
Signed-off-by: Greentime Hu <greentime.hu@sifive.com>
Signed-off-by: Vineet Gupta <vineetg@rivosinc.com>
[vineetg: create new patch from meshup, introduced asm variant]
Signed-off-by: Andy Chiu <andy.chiu@sifive.com>
[andy.chiu: remove calls from asm thus remove asm vaiant]