build target triple variable from sys/conf/newvers.sh

Retrieve FreeBSD revision number directly from sys/conf/newvers.sh
when building the compiler target triple value, avoiding manual
intervention on other files every new release.

Reviewed by: imp
MFC after: 2 months
Sponsored by: Instituto de Pesquisas Eldorado (eldorado.org.br)
Differential Revision: https://reviews.freebsd.org/D34429