[clang] [clang-repl] Fix PLT offset too large linker error on ARM (PR #78959)

[Martin Storsjö via cfe-commits]

mstorsjo wrote:

> > When cross compiling LLVM, I never have set `CMAKE_SYSTEM_PROCESSOR` so far, since we don't really have anything that uses it (before this), which means that this expands to an empty string. I guess I should set it still though.
> 
> Yes, I am just getting used to it as well. I think it's worth noting that this should be set in a toolchain file, because CMake seems to have special handling for them. When I pass `-DCMAKE_SYSTEM_PROCESSOR=ARM` at configuration time, it gets overwritten with my host arch.

I think that's (somewhat?) expected. When you're not cross compiling, cmake explicitly sets `CMAKE_SYSTEM_PROCESSOR` to `CMAKE_HOST_SYSTEM_PROCESSOR`, ignoring whatever you set manually. (Dunno if it would make any difference if you'd set it in a toolchain file.) Only if you're cross compiling (which CMake defines as if you're setting `CMAKE_SYSTEM_NAME`, regardless if cross compiling from Linux to Linux on another arch), it doesn't set it and passes whatever you set originally through.

This actually makes `CMAKE_SYSTEM_PROCESSOR` a bit problematic; if you're on x86_64 but targeting i386, or on aarch64 but targeting arm, you might not want to consider it a cross build (as you can execute the build products) but CMake then will hide whatever you're really targeting with the info gathered from the host environment.

https://github.com/llvm/llvm-project/pull/78959