doc: update linux supported versions

The libuv 1.12.0 update bumped the minimum supported version
of linux + glibc. This commit updates BUILDING.md to reflect
the new values.

PR-URL: https://github.com/nodejs/node/pull/13306


Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>