LLVM 20 transition

[ziyao233]

TODO

• Fix build of LLVM 20
• Rust compatibility
  • Update to LLVM 20 rust-lang/rust#135763
  • Is it necessary to wait for Rust 1.87.0? (will stablize on May 15th)
• Graphics stack
  • spirv-llvm-translator
  • mesa

Workarounds/Bugs to recheck after upgrading

• linux: Workaround for stack frame size of AMDGPU
  • linux: https://github.com/eweOS/packages/blob/linux/0001-amdgpu-dml2-Increase-max-stack-size.patch
  • linux-lts: https://github.com/eweOS/packages/blob/linux-lts/0001-amdgpu-dml2-Increase-max-stack-size.patch
• vala: ICE on riscv64
• OpenBLAS: ICE on loongarch64

[ziyao233]

Noted some warnings when building 20.1.2,

CMake Warning at CMakeLists.txt:161 (message):                                                                                                                 
  Using LLVM_ENABLE_PROJECTS=compiler-rt is deprecated now, and will become a                                                                                  
  fatal error in the LLVM 21 release.  Please use                                                                                                              
  -DLLVM_ENABLE_RUNTIMES=compiler-rt or see the instructions at                                                                                                
  https://compiler-rt.llvm.org/ for building the runtimes.    

we'd better to fix it in #3600 as well.