[klibc] Kbuild, klcc: Support multiple objects in KLIBCCRTSHARED

On some architectures (notably riscv64) we need a wrapper for the main
function that is statically linked even when using a shared library.

* Allow architectures to override KLIBCCRTSHARED (they should always
  append to it)
* Change klcc to use the list of filenames in KLIBCCRTSHARED, rather
  than assuming it's just interp.o

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
4 files changed