[OpenMP] Use weak attribute in interface only for static library

This is to address the issue reported at:
https://bugs.llvm.org/show_bug.cgi?id=46863

Since weak is meaningless for a shared library interface function, this patch
disables the attribute, when the OpenMP library is built as shared library.

ompt_start_tool is not an interface function, but a internally called function
possibly implemented by an OMPT tool.
This function needs to be weak if possible to allow overwriting ompt_start_tool
with a function implementation built into the application.

Differential Revision: https://reviews.llvm.org/D84871