[klibc] Move linker option probing to Kbuild.klibc

Compiler option probing is done in Kbuild.klibc, so move the check for
lld and the related option definition to there as well.

Also stop exporting LD_IMAGE_BASE_OPT, as it's only needed in the
architecture MCONFIG which is included here.

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