hadrian doesn't use inplace mingw toolchain on Windows

While looking at #18550 (closed), I noticed that Hadrian (and perhaps make) does not use the in-place mingw toolchain on Windows. Specifically, only builds that are run using ghc as the driver will benefit from the inplace toolchain.

Use of the in-place toolchain is achieved through logic in the FP_SETTINGS function in aclocal.m4, which sets SettingsCCompilerCommand, among other things. However, it does not set Hadrian's system-cc or make's CC and CC_STAGE0 variables. Consequently any builds (or linking jobs) which call the C compiler or linker directly will use the wrong toolchain on Windows. I wonder if this is in part to blame for our Windows path length issues: some of our builds aren't using the patched toolchain.

Fixing this is a bit tricky since GHC has special logic to resolve the $tooldir token that appears in SettingsCCompilerCommand and friends. I don't think we want to replicate this in Hadrian/make.