This commit moves the do_libc_configure function to do_libc_backend and
switches do_libc_start_files and do_libc_final to call do_libc_backend.
The major reason for the rewrite is that musl => 1.1.13 has had it's own
build system rewritten and can now build out-of-tree.
Signed-off-by: Bryan Hundven <bryanhundven@gmail.com>