MFH: r408340

devel/cargo: Build on amd64 only

Cargo requires a prebuilt bootstrapped version of itself. The only one
available for FreeBSD is for amd64.

This prevents build failures on i386.

Reviewed by: jbeich
Approved by: ports-secteam (feld), jbeich
Differential Revision: https://reviews.freebsd.org/D5219