Document CPUFLAGS here (it's already documented in share/mk/bsd.README).

This will address the initial concern in PR port-alpha/55545.

BUILDING

@@ -192,6 +192,11 @@ CONFIGURATION
                  information is available in the GCC documentation of
                  -frandom-seed.
 
+     CPUFLAGS    Additional flags to the compiler/assembler to select CPU
+                 instruction set options, CPU tuning options, etc.
+
+                 Default: Unset.
+
      DESTDIR     Directory to contain the built NetBSD system.  If set,
                  special options are passed to the compilation tools to
                  prevent their default use of the host system's /usr/include,

doc/BUILDING.mdoc

@@ -1,4 +1,4 @@
-.\"	$NetBSD: BUILDING.mdoc,v 1.130 2020/10/12 21:14:37 uwe Exp $
+.\"	$NetBSD: BUILDING.mdoc,v 1.131 2020/10/13 01:59:32 thorpej Exp $
 .\"
 .\" Copyright (c) 2001-2011 The NetBSD Foundation, Inc.
 .\" All rights reserved.
@@ -405,6 +405,11 @@ built from the same sources, resulting in identical (reproducible) builds.
 Additional information is available in the GCC
 documentation of -frandom-seed.
 .
+.It Sy CPUFLAGS
+Additional flags to the compiler/assembler to select
+CPU instruction set options, CPU tuning options, etc.
+.DFLTu
+.
 .It Sy DESTDIR
 Directory to contain the built
 .Nx