doc: update kernel and glibc reqs for PPCle

We've moved master to build on centos7 for PPCle, adjust
the glibc and kernel levels accordingly.

PR-URL: #28162
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Beth Griggs <Bethany.Griggs@uk.ibm.com>

Author: mhdawson

BUILDING.md

@@ -103,8 +103,8 @@ platforms. This is true regardless of entries in the table below.
 | GNU/Linux        | arm64            | kernel >= 4.5, glibc >= 2.17    | Tier 1       | e.g. Ubuntu 16.04, Debian 9, EL 7 <sup>[3](#fn3)</sup> |
 | GNU/Linux        | armv7            | kernel >= 4.14, glibc >= 2.24   | Tier 1       | e.g. Ubuntu 18.04, Debian 9       |
 | GNU/Linux        | armv6            | kernel >= 4.14, glibc >= 2.24   | Experimental | Downgraded as of Node.js 12       |
-| GNU/Linux        | ppc64le >=power8 | kernel >= 3.13.0, glibc >= 2.19 | Tier 2       | e.g. Ubuntu 16.04, EL 7           |
-| GNU/Linux        | s390x            | kernel >= 3.10.0, glibc >= 2.17 | Tier 2       | e.g. EL 7                         |
+| GNU/Linux        | ppc64le >=power8 | kernel >= 3.10.0, glibc >= 2.17 | Tier 2       | e.g. Ubuntu 16.04 <sup>[1](#fn1)</sup>, EL 7  <sup>[2](#fn2)</sup> |
+| GNU/Linux        | s390x            | kernel >= 3.10.0, glibc >= 2.17 | Tier 2       | e.g. EL 7 <sup>[2](#fn2)</sup>    |
 | Windows          | x64, x86 (WoW64) | >= Windows 7/2008 R2/2012 R2    | Tier 1       | <sup>[4](#fn4),[5](#fn5)</sup>    |
 | Windows          | x86 (native)     | >= Windows 7/2008 R2/2012 R2    | Tier 1 (running) / Experimental (compiling) <sup>[6](#fn6)</sup> | |
 | Windows          | arm64            | >= Windows 10                   | Experimental |                                   |
@@ -165,7 +165,7 @@ Binaries at <https://nodejs.org/download/release/> are produced on:
 | darwin-x64 (and .pkg) | macOS 10.11, Xcode Command Line Tools 8 with -mmacosx-version-min=10.10  |
 | linux-arm64           | CentOS 7 with devtoolset-6 / GCC 6                                       |
 | linux-armv7l          | Cross-compiled on Ubuntu 16.04 x64 with [custom GCC toolchain](https://github.com/rvagg/rpi-newer-crosstools)   |
-| linux-ppc64le         | Ubuntu 14.04 with GCC 6                                                  |
+| linux-ppc64le         | CentOS 7 with devtoolset-6 / GCC 6 <sup>[7](#fn7)</sup>                  |
 | linux-s390x           | RHEL 7 with devtoolset-6 / GCC 6 <sup>[7](#fn7)</sup>                    |
 | linux-x64             | CentOS 7 with devtoolset-6 / GCC 6 <sup>[7](#fn7)</sup>                  |
 | sunos-x64             | SmartOS 18 with GCC 7                                                    |