doc: documenting a bit more FreeBSD case

FreeBSD provides more up to date compilers than
the one provided by the system.

Author: devnexen

BUILDING.md

@@ -119,7 +119,7 @@ platforms. This is true regardless of entries in the table below.
 | macOS            | x64              | >= 10.11                        | Tier 1       |                                   |
 | SmartOS          | x64              | >= 18                           | Tier 2       |                                   |
 | AIX              | ppc64be >=power7 | >= 7.2 TL02                     | Tier 2       |                                   |
-| FreeBSD          | x64              | >= 11                           | Experimental | Downgraded as of Node.js 12       |
+| FreeBSD          | x64              | >= 11                           | Experimental | Downgraded as of Node.js 12  <sup>[7](#fn7)</sup>     |
 
 <em id="fn1">1</em>: GCC 6 is not provided on the base platform, users will
   need the
@@ -153,6 +153,10 @@ are provided. However, tests in our infrastructure only run on WoW64.
 Furthermore, compiling on x86 Windows is currently considered Experimental and
 may not be possible.
 
+<em id="fn7">7</em>: FreeBSD 12.0 system's compiler is clang 6.0.1 but
+FreeBSD 12.1's is the 8.0.1 version. In both cases, other LLVM versions
+are provided via the system's package manager up to 9.0.
+
 ### Supported toolchains
 
 Depending on the host platform, the selection of toolchains may vary.