Skip the CPU detection on MinGW like MSVC for amd64

[illwieckz]

Extracted from:

• Make the loader code compatible with MinGW (2) #17

Replaying parts of:

• Prepare for the CMake plumbing to build the loader with MinGW #7
• Make the loader code compatible with MinGW #12

Does nothing until:

• Make the loader code compatible with MinGW (2) #17
• Prepare for the CMake plumbing to build the loader with MinGW (2) #18
• CMake plumbing to build the loader (2) #14

[slipher]

Microsoft longjmp documentation warns:

• Don't use longjmp to transfer control out of an interrupt-handling routine unless the interrupt is caused by a floating-point exception. In this case, a program may return from an interrupt handler via longjmp if it first reinitializes the floating-point math package by calling _fpreset.

• Don't use longjmp to transfer control from a callback routine invoked directly or indirectly by Windows code.

Anyway as discussed elsewhere, since 32-bit Windows is obsolete it's probably not worth trying to port this to MinGW.

[illwieckz]

Actually the code is already entirely skipped on amd64 with MSVC:

#if defined(NACL_WINDOWS_MSC_64)
static int CheckCPUFeatureDetection(NaClCPUFeaturesX86 *cpuf) {
  /* Unfortunately the asm_ tests will not work on 64-bit Windows */
  return 0;
}
#else

So actually my second commit was adding with MinGW something that was even not implemented with MSVC (on amd64).

So we can just decide to just always skip the code on MinGW, which is my first commit.

[illwieckz]

So I removed the second commit. MinGW now does like MSVC amd64: skip the tests.

We already know we can skip the tests since MSVC does it in all cases with amd64.

[illwieckz]

It happened that it was already skipped on MinGW amd64 too because it was just testing for windows-amd64, but now this also skips on MinGW i686. Skipping a code that is known to be skippable is good enough maintenance for an obsolete platform.

[illwieckz]

To quote the remaining commit message:

win32: skip some CPU detection on Windows with MinGW like amd64 MSVC

The only known working implementation is for i686 MSVC.

- detection was already disabled and marked at not working with amd64 MSVC,
- detection was already disabled on amd64 MinGW because amd64 Windows was tested, not just amd64 MSVC,
- there is no MinGW implementation in the code,
- this disables MinGW detection explicitely whatever the architecture.

[illwieckz]

@slipher I need this to get the i686 MinGW build working.

[illwieckz]

I added a patch to silence a warning.

[slipher]

@slipher I need this to get the i686 MinGW build working.

I thought you gave up on using MinGW for now since the ones in Debian are too old to build it (needs GCC 16; also may depend on the right TLS flavor being configured).

In view of Unvanquished/Unvanquished#3572 maybe we will never need to build an x86 MinGW version.