Previously, we would precompile D test dependencies using the
host's dtrace, which unconditionally outputs ELF files in the
host's format. This breaks the cross-compile build with errors
like the following:

dtrace: failed to link script: incorrect ELF machine type for
object file: tst.usdt.pieo

• usdt.o ---
• Failed target: usdt.o

This patch moves compilation to runtime for all C-based testcases
that have a dependent D source file.