Fix openmp for xcode.

[Jarod42]

What does this PR do?

pass -fopenmp in xcode generator when openmp "On" is provided

How does this PR change Premake's behavior?

xcode generator only

Anything else we should know?

MacOSX seems to need special install of gcc to have that flag accepted.
I didn"t have courage to do it for premake-sample-projects
so https://github.com/Jarod42/premake-sample-projects/actions/runs/2469814949 still fails my test
but flag is present in command line.

Did you check all the boxes?

• Focus on a single fix or feature; remove any unrelated formatting or code changes
• Add unit tests showing fix or feature works; all tests pass
• Mention any related issues (put closes #XXXX in comment to auto-close issue when PR is merged)
• Follow our coding conventions
• Minimize the number of commits
• Align documentation to your changes

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[samsinsane]

LGTM, can you also update the docs for the openmp API to list XCode support in beta2?

[Jarod42]

Doc updated.

[Jarod42]

Failing tests on macosx seems to be (temporary) connexion errors.
unrelated to the PR.

[  FAILED  ] 2 tests, listed below:
[  FAILED  ] vs2010_ensure_nuget_imports.structureIsCorrect
[  FAILED  ] vs2010_extension_targets.addsImport_onEachNuGetPackage

** Error: NuGet API error (0)
SSL connect error
unction 'testFunction'