[20.x] GCC build and test fixes for offload

[mgorny]

commit 689ef5fda0ab07dfc452cb16d3646d53e612cb75
Author:     Michał Górny <mgorny@gentoo.org>
AuthorDate: 2025-02-02 16:55:39 +0100
Commit:     GitHub <noreply@github.com>
CommitDate: 2025-02-02 16:55:39 +0100

    [offload] [test] Use test compiler ID rather than host (#124408)
    
    Use the test compiler ID to verify whether tests can be run rather than
    the host compiler. This makes it possible to run tests (with Clang)
    while the library itself was built with GCC.

commit 359a9131704277bce0f806de31ac887e68a66902
Author:     Michał Górny <mgorny@gentoo.org>
AuthorDate: 2025-02-02 16:55:22 +0100
Commit:     GitHub <noreply@github.com>
CommitDate: 2025-02-02 16:55:22 +0100

    [offload] `gnu::format` with variadic template functions is Clang-only (#124406)
    
    Use `gnu::format` attribute only when compiling with Clang, as using it
    against variadic template functions is a Clang extension and is not
    supported by GCC.
    
    See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77958
    
    Fixes #119069

[mgorny]

/cherry-pick 359a913 689ef5f

[llvmbot]

/pull-request #125498

[llvmbot]

@llvm/issue-subscribers-offload

Author: Michał Górny (mgorny)

``` commit 689ef5f Author: Michał Górny <mgorny@gentoo.org> AuthorDate: 2025-02-02 16:55:39 +0100 Commit: GitHub <noreply@github.com> CommitDate: 2025-02-02 16:55:39 +0100

[offload] [test] Use test compiler ID rather than host (#<!-- -->124408)

Use the test compiler ID to verify whether tests can be run rather than
the host compiler. This makes it possible to run tests (with Clang)
while the library itself was built with GCC.

commit 359a913
Author: Michał Górny <mgorny@gentoo.org>
AuthorDate: 2025-02-02 16:55:22 +0100
Commit: GitHub <noreply@github.com>
CommitDate: 2025-02-02 16:55:22 +0100

[offload] `gnu::format` with variadic template functions is Clang-only (#<!-- -->124406)

Use `gnu::format` attribute only when compiling with Clang, as using it
against variadic template functions is a Clang extension and is not
supported by GCC.

See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77958

Fixes #<!-- -->119069

</details>