devel/cargo-c: fix build on armv7

The address space does not suffice to build cargo-c on armv7 with LTO.
Disable LTO on armv6/armv7 to make the build succeed.

Approved by: portmgr (build fix blanket)
MFH: 2023Q4

(cherry picked from commit 77455882b6f8c9854c788f45d9fb4abd93555647)