This PR replaces the few uses of global.params.is<OS> with
global.params.targetTriple.isOS<OS>(). This avoids adding a new
boolean for each supported OS.
It also defines all xBSD-type OS as using the dso-registry.
In case of a singleobj build and a provided output file (.exe or .obj).
The dmd-testsuite tests used to produce lots of .o files while expecting
.obj ones as specified in the command line.