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 510403c1)