doc: add OS X to instead of only macOS

The last OS X version is 10.11 El Captian, and the first macOS version
is 10.12 Sierra. So >= 10.8 < 10.10 is called OS X and >= 10.10 is
called OS X or macOS.

Refs: https://en.wikipedia.org/wiki/MacOS#Release_history

PR-URL: nodejs/node#21033
Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Yorkie Liu <yorkiefixer@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

BUILDING.md

@@ -41,14 +41,14 @@ platforms in production.
 |--------------|--------------|----------------------------------|----------------------|------------------|
 | GNU/Linux    | Tier 1       | kernel >= 2.6.32, glibc >= 2.12  | x64, arm             |                  |
 | GNU/Linux    | Tier 1       | kernel >= 3.10, glibc >= 2.17    | arm64                |                  |
-| macOS        | Tier 1       | >= 10.10                         | x64                  |                  |
+| macOS/OS X   | Tier 1       | >= 10.10                         | x64                  |                  |
 | Windows      | Tier 1       | >= Windows 7/2008 R2/2012 R2     | x86, x64             | vs2017           |
 | SmartOS      | Tier 2       | >= 15 < 16.4                     | x86, x64             | see note1        |
 | FreeBSD      | Tier 2       | >= 10                            | x64                  |                  |
 | GNU/Linux    | Tier 2       | kernel >= 3.13.0, glibc >= 2.19  | ppc64le >=power8     |                  |
 | AIX          | Tier 2       | >= 7.1 TL04                      | ppc64be >=power7     |                  |
 | GNU/Linux    | Tier 2       | kernel >= 3.10, glibc >= 2.17    | s390x                |                  |
-| macOS        | Experimental | >= 10.8 < 10.10                  | x64                  | no test coverage |
+| OS X         | Experimental | >= 10.8 < 10.10                  | x64                  | no test coverage |
 | GNU/Linux    | Experimental | kernel >= 2.6.32, glibc >= 2.12  | x86                  | limited CI       |
 | Linux (musl) | Experimental | musl >= 1.0                      | x64                  |                  |
 
@@ -408,7 +408,7 @@ This version of Node.js does not support FIPS.
 It is possible to specify one or more JavaScript text files to be bundled in
 the binary as builtin modules when building Node.js.
 
-### Unix / macOS
+### Unix/macOS
 
 This command will make `/root/myModule.js` available via
 `require('/root/myModule')` and `./myModule2.js` available via