Update the build_sysroot to build an LLVM 11 sysroot, dealing with
LLVM's change in structure. Also re-add peru.yaml for getting a
pre-built sysroot via peru.
See: https://github.com/buildinspace/peru
* Link host-targeted binaries with lld
* Add peru script for getting prebuilt sysroot
* Add readme for prebuilt sysroots
* Remove non-working build_sysroot_gcc.sh, rename clang version to just
build_sysroot.sh