Enable RVV support in CMake and Bazel for RISC-V targets

Enable RISC-V Vector (RVV) by default when building natively on RISC-V
platforms (such as SpacemiT K1/K3) using CMake and Bazel:
- CMakeLists.txt: Detect riscv64 and riscv32, probe compiler support
  for -march=rv64gcv / -march=rv32gcv, and append architecture flags
  with auto-vectorization disabled (-fno-vectorize -fno-slp-vectorize).
  Support custom architecture flags via -DRISCV_COMPILER_FLAGS and
  provide -DUSE_RVV=OFF to disable.
- BUILD.bazel: Add RVV_COPTS for @platforms//cpu:riscv64 and
  @platforms//cpu:riscv32 with -fno-vectorize.
- docs/getting_started.md: Document native SBC builds, custom targets
  (SiFive X280, Coral 32-bit Zve32x), Bazel, and Blaze commands.

Test: cmake build on host and simulated riscv64; clang cross-compile tests
Bug: none
Change-Id: Iecb20b73b2934b6477e7e0c71b3539a8251e4e8b
TAG=agy
CONV=2f28dd8a-c892-4c29-8aed-606afd090398
Reviewed-on: https://chromium-review.googlesource.com/c/libyuv/libyuv/+/8349493
Reviewed-by: richard winterton <rrwinterton@gmail.com>
Commit-Queue: Frank Barchard <fbarchard@google.com>