[Enhancement] MinGW toolchain based on the UCRT

[d-musique]

The MinGW-w64 12.x compiler toolchain, which is not currently available as a package, has introduced the support of Universal CRT.

See: https://github.com/mingw-w64/mingw-w64/blob/master/mingw-w64-doc/howto-build/ucrt-vs-msvcrt.txt

This feature improves on the old MSVCRT-based toolchain, by complying with the newer standards of the C language, and it makes the compiled binaries interoperable with other compilers, notably the proprietary one from MS.
The UCRT is the new default starting from MinGW-w64 12.x.

This update requires packaging effort because the specs need significant changes, possibly a reworked bootstrap.

https://github.com/OpenMandrivaAssociation/gcc
https://github.com/OpenMandrivaAssociation/mingw

A UCRT compiler has a target triple in the form x86_64-w64-mingw32ucrt.
There appears to be two ways to go from here:

1. Introduce mingw32ucrt as a new cross compile target in gcc, and then introduce ucrt specific packages (libc), while keeping non-ucrt variants available
2. Get rid of legacy mingw completely in favor of ucrt, and have ucrt support packages only

The "ucrt-ness" of the compiler and the runtimes go together.

It can be investigated whether clang is an option for building the libc and crt instead of gcc, since clang has support of a vast set of architectures, which includes the ucrt-based mingw.

[d-musique]

Before anything, mingw should be updated from version 11 to version 13.

For this build to pass, it has to move from a 2 step build to a 3 step build.
headers (aka bootstrap) → crt → libraries

The easy solution is to move the libraries (winpthread + a few others) out of *-libc and into another rpm spec.
These libraries (mingw-w64-libraries) are extra goodies which are mingw-specific and not really "libc".

Is this approved?

[d-musique]

The MinGW progress currently is:

• update to 13.0.0, which introduces the fundamentals to be able to link ucrt-based programs
• gcc support missing, more precisely the language runtimes

As it is, one can manage to build proper C programs by passing a few options -mcrtdll=ucrt -D_UCRT.
This is not true of C++, which requires its runtime rebuilt, which is part of the gcc package.

In order to complete the task, there are two steps remaining

• rebuild package gcc with ucrt on (if possible without modifying the target triplet? needs more info)
• rebuild package mingw using --with-default-msvcrt=ucrt

[ZeroAbility]

It can be investigated whether clang is an option for building the libc and crt instead of gcc, since clang has support of a vast set of architectures, which includes the ucrt-based mingw.

Since this is our default toolchain, that is probably where the bulk of the effort should go.

[d-musique]

There's a few things I want to be clear about before moving forward.

• There is an ABI break. Code built with the old cross-compiler will not be compatible. Probably not a big issue because programs targeting on this target usually bundle all their libs.
• Programs built with this new runtime will not be able to run on older versions of OS (prior to XP)
• This compiler will find itself temporarily in a semi-broken state, until the full gcc rebuild is completed

For info, my findings were that 2 elements were needed to cause the compiler to transparently select either runtime.

• the definition __MSVCRT_VERSION__ in _mingw.h.in
• which of 2 libraries mingw installs under the name libmsvcrt.a (gcc specs have the link hardcoded as -lmsvcrt)