[lldb/testsuite] Modernize 2 test Makefiles

Those old Makefiles used completely ad-hoc rules for building files,
which means they didn't obey the test harness' variants.

They were somewhat tricky to update as they use very peculiar build
flags for some files. For this reason I was careful to compare the
build commands before and after the change, which is how I found the
discrepancy fixed by the previous commit.

While some of the make syntax used here might not be easy to grasp for
newcomers (per-target variable overrides), it seems better than to
have to repliacte the Makefile.rules logic for the test variants and
platform support.