riscv: Introduce struct/helpers to save/restore per-task Vector state

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

This include Vector Regfile and CSRs holding dynamic configuration state
(vstart, vl, vtype, vcsr). The Vec Register width could be implementation
defined, but same for all processes, so that is saved separately.

This is not yet wired into final thread_struct - will be done when
__switch_to actually starts doing this in later patches.

Given the variable (and potentially large) size of regfile, they are
saved in dynamically allocated memory, pointed to by datap pointer in
__riscv_v_ext_state.

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: merged bits from 2 different patches]
Signed-off-by: Andy Chiu <andy.chiu@sifive.com>
[andy.chiu: use inline asm to save/restore context, remove asm vaiant]