Cannot produce a Clang/Musl SDK

[deciduously]

Clang-based Linux SDKs and musl-gcc SDKs are both supported, but the current implementation fails to compile clang using a musl sysroot. It successfully builds stage 1, and then stage2 configure fails:

CMake Error at cmake/modules/HandleLLVMOptions.cmake:365 (message):
  Host compiler does not support '-fuse-ld=lld'.  Please make sure that 'lld'
  is installed and that your host compiler can compile a simple program when
  given the option '-fuse-ld=lld'.

This is surprising as at this point, the host compiler is the clang we just built, which certainly should support -fuse-ld=lld.