riscv: Prepare vector struct and helpers

Add vector state context struct to be added later in thread_struct. And
prepare low-level helper functions to save/restore vector contexts.

Vector state includes vector reg file and additional dynamic
configuration CSRs. To handle variable sized reg file context (due to
implementation defined ref size) and to enable lazy-allocation of this,
there's datap which points to appropriate location on user/kernel mode
stack as relevant

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: Andy Chiu <andy.chiu@sifive.com>