Build fails with external LLVM with ARM backend disabled

[pcercuei]

When compiling with an external LLVM toolchain (using --llvm-root) that does not support the ARM backend, the compilation of the Rust compiler will fail.

I traced it to mk/main.mk:270:

LLVM_COMPONENTS=x86 arm aarch64 mips powerpc ipo bitreader bitwriter linker asmparser mcjit interpreter instrumentation

This causes the link step to fail, as "llvm-config" issues an error as it does not recognize the "arm" and "aarch64" components.

[pcercuei]

Actually this is not enouch since the LLVM_LIBS_* and LLVM_LDFLAGS_* are set in mk/main.mk but don't seem to be used elsewhere.
When adding those variables inside mk/target.mk for the linking step, I still get errors as Rustc seems to use ARM-specific symbols from the LLVM libraries.

[tamird]

Dupe of #14729?

[alexcrichton]

Looks like it!