When the bootstrap code builds LLVM, it sometimes sets the CMake variable CMAKE_EXE_LINKER_FLAGS. For example, when instructed to also build the LLVM tools:

When CMAKE_EXE_LINKER_FLAGS is unset, CMake initializes it to LDFLAGS, but it does not consider LDFLAGS if CMAKE_EXE_LINKER_FLAGS is set. This means that Rust's LLVM bootstrap will sometimes respect (potentially required) LDFLAGS and sometimes not depending on a number of unexpected factors, such as whether llvm tools are also being built, whether we're building for a non-FreeBSD RISC-V target, or if we're building for powerpc-unknown-freebsd. This, in turn, makes it difficult to bootstrap Rust in situations where additional linker flags are required, such as if dependencies are not stored in standard system-wide location such as /usr/lib.

I propose that anywhere we set CMAKE_EXE_LINKER_FLAGS, we append LDFLAGS from the environment to the value to retain parity with the behavior when the configuration isn't set.