Roll up msys2/clang/windows build fixes (#59003)
This rolls up everything I had to change to get a successful source
build of Julia under msys2. It's a misc collection of msys2, clang and
other fixes. With this, I can use the following Make.user:
```
USE_SYSTEM_CSL=1
USE_BINARYBUILDER_LLVM=0
CC=clang
CXX=clang++
FC=gfortran
```
The default USE_SYSTEM_CSL is broken due to #56840 With
USE_SYSTEM_CSL=1, LLVM is broken due to #57021 Clang is required because
gcc can't do an LLVM source build due to known export symbol size limits
(ref JuliaPackaging/Yggdrasil#11652).
That said, if we address the ABI issues in #56840, the default Make.user
should build again (with BB-provided LLVM).
