[doc] How to cross compile v8 for riscv32 architecture on x86_64 linux

[VitalyAnkh]

Prerequisite: you are on a up-to-date x86_64 linux environment.

1. download riscv32 gnu gcc toolchain: https://github.com/riscv-collab/riscv-gnu-toolchain/releases/download/2022.06.10/riscv32-glibc-ubuntu-20.04-nightly-2022.06.10-nightly.tar.gz, then extract them to /opt/riscv/. Your /opt/riscv should look like this (ignore the riscv64-unknow-linux-gnu entry, it's present because I also installed the gnu gcc riscv64 toolchain):
   
2. cd to your v8 directory, then cd build. The build directory contains toolchain configurations for building v8. Download this patch from @luyahan: https://gist.github.com/VitalyAnkh/6e064ab3dcdf1be1032111bc8fa61acb. Then git apply <path-to-the-patch>.
3. Edit v8/build/config/riscv.gni, assign values to riscv_gcc_toolchain_path and riscv_sysroot, like this
   
4. cd to your v8 directory, run gn gen out/riscv32.native.debug --args='is_component_build=false is_debug=true target_cpu="riscv32" v8_target_cpu="riscv32" use_goma=false goma_dir="None" treat_warnings_as_errors=false is_clang=false', then ninja -C out/riscv32.native.debug -j12.

Enjoy hacking!