Make Coverage checking more stable

[wilzbach]

After @MartinNowak's nice work at dlang/dmd#6022, here's a similar setup for Druntime.

Short summary

• Travis clogs our PR queue -> use CircleCi for coverage testing
• The coverage testing script now allows to run for 64-bit & 32-bit (a run takes only 3 minutes, so I think we can enable here two containers too)
• moved style check to the posix.mak as style target

[dnadlinger]

Leaving for @MartinNowak to greenlight,but should be okay.

[wilzbach]

moved style check to the posix.mak as style target

Oh sorry ... checkwhitespace was already a defined target in the Makefile, so I changed to style require it. style will be used for a similar PR at Phobos, s.t. we have clear naming and the style target at druntime could/should be expanded anyhow ;-)

[MartinNowak]

Auto-merge toggled on

[MartinNowak]

Can we also please enable CircleCI for druntime and Phobos @WalterBright @andralex.
https://circleci.com/add-projects

[andralex]

@MartinNowak are we good now?

[wilzbach]

@MartinNowak are we good now?

Yep we are all set & all projects are enabled, only the configuration for Phobos is pending
See it in "live" e.g here:

#1602