Move riscv64 cross builds to clang

[sxa]

nodejs/build#4091 covers moving some of the main CI test machines across to using clang instead of gcc as V8 stopped formally supported GCC in September 2024 starting with V8 14

We should look at the feasibility of moving the RISC-V build to clang. If this can be done then we can probably replicate it the process for some of the other unofficial builds if desired. I'll also note that on my personal RISC-V systems building with clang seemed to use less memory allowing for higher -j values to be used in the build so there is a potential advantage on that basis in the future from making this switch.

[sxa]

I'm currently testing with builds in the following dockerfile:

FROM docker.io/library/ubuntu:25.10
RUN apt -y update
RUN apt install -y clang lld gcc-riscv64-linux-gnu g++-riscv64-linux-gnu make git
RUN apt install -y file xz-utils less bzip2

(Stuff in the second line are not all required for the build but I've included them because I find them useful :-) )
Environment being used:

export CXX='ccache clang++ --target=riscv64-linux-gnu'
export CC='ccache clang --target=riscv64-linux-gnu'
export CC_host='ccache clang'
export CXX_host='ccache clang++'
./configure --openssl-no-asm --cross-compiling --dest-cpu=riscv64 --use_clang

The resulting build completed but resulted in an illegal instruction crash when I tried to run it on my SpacemiT-K1 board - probably incorrect default architecture flags when cross-compiling with clang. Will try again but it's good that it produced something :-)

[sxa]

Using LTS version of Ubuntu is also good, and forcing -march=rv64gc on the compiler flags seems to work:

FROM docker.io/library/ubuntu:24.04
RUN apt -y update
RUN apt install -y clang-19 lld gcc-riscv64-linux-gnu g++-riscv64-linux-gnu
RUN apt install -y make git ccache file xz-utils less bzip2 joe

export export CC='ccache clang-19  --target=riscv64-linux-gnu -march=rv64gc'
export CXX='ccache clang++-19  --target=riscv64-linux-gnu -march=rv64gc'
export CC_host='ccache clang-19'
export CXX_host='ccache clang++-19'
./configure --openssl-no-asm --cross-compiling --dest-cpu=riscv64 --use_clang

Noting that the clang 20 in Ubuntu 25.10 from the dockerfile above is also ok when -march=rv64gc is set.

[sxa]

(I also tried on Fedora but is a bit more tricky due to the lack of a package with the sysroot. I have not yet got it working by copying the sysroot from Ubuntu, but we will likely stick with ubuntu anyway as that's what we're using in the currnet recipes)