• RVV options are moved to configuration scripts instead of toolchains

cmake ... -DCPU_BASELINE=RVV

Extra cmake options:

• RISCV_RVV_SCALABLE: ON/OFF
• redefine CPU_RVV_FLAGS_ON if -march=rv64gcv should be replaced e.g. for -march=rv64gcv_zfh

List of changes:

• riscv64-071-gcc.toolchain.cmake - no changes (deprecated)
• riscv64-clang.toolchain.cmake - dropped v to avoid forcing of RVV, moved out SCALABLE option from toolchain
• riscv64-gcc.toolchain.cmake - dropped v to avoid forcing of RVV
• riscv64-gnu.toolchain.cmake - dropped on favor of riscv-gnu.toolchain.cmake
• riscv.toolchain.cmake - fixed multiple issues and renamed to riscv-gnu.toolchain.cmake

allow_multiple_commits=1
force_builders=Custom
Xbuild_image:Custom=riscv-gcc
build_image:Custom=riscv-gcc-rvv
Xbuild_image:Custom=riscv-clang
Xbuild_image:Custom=riscv-clang-rvv
Xbuild_image:Custom=riscv-clang-rvv-128
test_modules:Custom=core,imgproc,dnn
buildworker:Custom=linux-1
test_timeout:Custom=600
build_contrib:Custom=OFF