Divorce the ARM bare-metal GCC ports from the FreeBSD GCC xtoolchain ports.

[jhb]

Summary

In the past, changes to the FreeBSD GCC xtoolchain ports have broken the
ARM bare-metal GCC ports by accident. In addition, we'd like to move to
having multiple FreeBSD GCC xtoolchain ports for different GCC versions.
It seems best if we make the two groups of ports independent so that changes
to one do not affect the other.

Test Plan

• poudriere bulk -t of all of the affected GCC ports

I would suggest that someone in the ARM world take over maintainership
of the two ARM baremetal ports.