After commit a50082c1 we use -ffunction-sections -fdata-sections
for all C compilations, when $1_$2_SplitSections is set. But that
variable was set in build-package.mk which is not run for the RTS.
As a result the RTS was not being split, leading to larger binaries.

This commit fixes RTS splitting by moving the definition of
$1_$2_SplitSections to distdir-opts.mk, which is run for the RTS
(and also from build-package.mk).