More incremental fixes from the build-infra project. The list of fixes include:

* More ExecuteWithLog.
* Native buildtools fixes.
* Make DTDParser quiet.
* LIBM cleanups.
* Export LDFLAGS_HASH_STYLE.
* Remove superfluous -export on LDFLAGS_windows. (JNIEXPORT used)
* Don't let make reconfigure delete spec.gmk on abort