When building some of the boot loaders with clang, and DEBUG_FLAGS or
CFLAGS having '-g' in it, clang outputs several assembly directives that
are too new for our version of binutils.
Therefore, assemble the resulting .s files with clang instead. A more
general solution can be implemented when a GNU as-compatible driver for
clang's integrated assembler appears.
Tested by: gjb
56af34a743