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

[Stefan Gränitz via cfe-commits]

weliveindetail wrote:

CMake does always have more surprises to offer :) Thanks for fixing it right away @mstorsjo!

> 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.

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