This adds support for the MCU psABI in a way different from r251223 and r251224, basically reverting most of these two patches.
The problem with the approach taken in r251223 is that it only handled libcalls that originated from the backend. However, the midend also inserts quite a few libcalls and - with good reason - assumes these use the platform's default calling convention.
The previous patch tried to insert inregs when necessary both in the FE and, somewhat hackily, in the CG. This patch (and its clang companion patch) goes in a different direction. It defines a new default calling convention for the MCU, which doesn't use inreg marking at all, similarly to what, say, x86-64 does.