Fix Intel Xcode builds with assembly

[gjasny]

When forcing the source file language to C, Xcode enforces the file to be compiled as C by appending -x c to the compiler command line.

For now try to limit the damage and only enforce the language if the ASM and C compilers differ.

Reproducer (CMake 3.26.4, Xcode 14.3):

cmake -S build/cmake -B _b -GXcode -DCMAKE_OSX_ARCHITECTURES=x86_64
cmake --build _b

Fix: #3622

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[gjasny]

I signed the CLA.

[Cyan4973]

The fix looks simple enough to me.
It's just a pity we don't have a CI test that would produce the error you've been experiencing,
this would ensure that the issue is really fixed, and more importantly, that it will remain fixed in future modifications.
That being said, some tests are more difficult to deploy in CI, and for XCode specifically, I have no idea.

@terrelln , any comment on this PR ?

[cfnptr]

All checks passed. Looks like we can merge this fix?

[Cyan4973]

All checks passed because there is no check that exercises the code path that's added in this commit.

That being said, no proposal emerged to create a relevant test, and after so much time, I doubt it ever will.
So I'll let it pass, and merge,
keeping in mind that an untested property is a property that might be silently lost in a future update.

[cfnptr]

@Cyan4973, I think we can add an Xcode runner to catch this kind of bugs. Apple allows building Xcode projects from the command line: https://developer.apple.com/library/archive/technotes/tn2339/_index.html.

Something like this:

1. cmake -G Xcode -DCMAKE_BUILD_TYPE=Release -S build/cmake/ -B build-xcode/
2. xcodebuild -project build-xcode/zstd.xcodeproj -scheme ALL_BUILD build