Fix nvc++ compiler warnings

[mhoemmen]

When I build the current stable branch (760060059fb746018a9849234e02dc9bf003861b) of the reference mdspan implementation (https://github.com/kokkos/mdspan) with nvc++ (either 22.7 or a pre-release version), I get a couple build warnings like the following.

".../tests/googletest-src/googletest/src/gtest-internal-inl.h", line 636: warning: unknown attribute "optimize" [unrecognized_attribute]
        GTEST_NO_INLINE_ GTEST_NO_TAIL_CALL_;
                         ^

Issue https://github.com/google/googletest/issues/3849 explains: GTest is using #elif __GNUC__ to protect use of __attribute__((optimize("no-optimize-sibling-calls"))). Many compilers that are not GCC define __GNUC__ to express some degree of compatibility with GCC extensions. This includes nvc++. nvc++ does not understand this attribute, so it emits a warning. My fix just disables use of that attribute if __NVCOMPILER is defined.

I've tested this fix with a pre-release (post-22.7) version of nvc++. It prevents the warning, and the reference mdspan implementation's tests build and run correctly.

Fixes #3849.

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[mhoemmen]

I think we have a CLA -- I'll have to look into this.