net/mpich: Fix build error on i386 CURRENT

A workaround existed in net/mpich for a missing feature in llvm 17.0.
The feature has been added to llvm 18.1 except for the i386 platform.
This patch adds llvm 18.1 i386 to the existing workaround.

PR: 280719
Approved by: submitter is maintainer

(cherry picked from commit 510403c1998e0f3b3890a97333f6b654db94beec)