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: #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>

Author: cjihrig

BUILDING.md

@@ -35,7 +35,7 @@ Support is divided into three tiers:
 
 |  System      | Support type | Version                          | Architectures        | Notes            |
 |--------------|--------------|----------------------------------|----------------------|------------------|
-| GNU/Linux    | Tier 1       | kernel >= 2.6.18, glibc >= 2.5   | x86, x64, arm, arm64 |                  |
+| GNU/Linux    | Tier 1       | kernel >= 2.6.32, glibc >= 2.12  | x86, x64, arm, arm64 |                  |
 | macOS        | Tier 1       | >= 10.10                         | x64                  |                  |
 | Windows      | Tier 1       | >= Windows 7 / 2008 R2           | x86, x64             | vs2015 or vs2017 |
 | SmartOS      | Tier 2       | >= 15 < 16.4                     | x86, x64             | see note1        |