devel/libunwind: link with -lgcc on armv7

This is required to get the __aeabi_uidivmod symbol. Random ports that
use libunwind fail to build or work if that symbol is not present.

Discussed with: andrew
Approved by: portmgr (build fix blanket)
MFH: 2025Q1

(cherry picked from commit 40cc5ff704801a2cb9e44a1e4bfb580878dd5494)