[ELF] --fortran-common: prefer STB_WEAK to COMMON

The ELF specification says "The link editor honors the common definition and
ignores the weak ones." GNU ld and our Symbol::compare follow this, but the
--fortran-common code (D86142) made a mistake on the precedence.

Fixes https://bugs.llvm.org/show_bug.cgi?id=51082

Reviewed By: peter.smith, sfertile

Differential Revision: https://reviews.llvm.org/D105945