Recent improvements to incremental compilation vastly improved the build times for JDK modules. The build times for microbenchmarks could use a similar improvement.

To reproduce:
- configure with jmh (microbenchmarks won't be built without jmh)
- build microbenchmarks, for example with this command line:
make test TEST=jaxp:tier1
- change one of the microbenchmark files
- perform another build with:
 make test TEST=jaxp:tier1 LOG=profile

This creates a build-profile.log file that details the time spent on every build command. On my machine the compilation of microbenchmarks took 78 seconds, which suggests that it's doing much more work than just recompiling one file.

The command used to compile benchmarks can be found here:
build/<conf>/support/test/micro/classes/_the.BUILD_JDK_MICROBENCHMARK_batch.cmdline.