biology/iqtree: Remove USES=compiler:openmp, fix amd64 clang++ build

• USES=compiler:openmp still forces building with GCC. This is no longer necessary on tier 1 platforms, which have openmp in base.

• Mark broken on i386, because base clang doesn't have 64 bit atomic functions for i386 (see bug 230888).

PR: 247753, 252379
Submitted by: cmt (based on)