toolchain: gcc: update 7.x to 7.2.0

Bump gcc from 7.1 to 7.2

Compile & run tested: ar71xx

Trace history of current patches and update with commit ref & comment
to give more clue as to why they're still around/needed.  Some have
changed form since the original commit but some clue is better than no
clue at all.

Signed-off-by: Kevin Darbyshire-Bryant <kevin@darbyshire-bryant.me.uk>
Tested-by: Koen Vandeputte <koen.vandeputte@ncentric.com>

toolchain/gcc/patches/7.2.0/920-specs_nonfatal_getenv.patch

@@ -0,0 +1,22 @@
+Author: Jo-Philipp Wich <jow@openwrt.org>
+Date:   Sat Apr 21 03:02:39 2012 +0000
+
+    gcc: add patch to make the getenv() spec function nonfatal if requested environment variable is unset
+    
+    SVN-Revision: 31390
+
+--- a/gcc/gcc.c
++++ b/gcc/gcc.c
+@@ -9280,8 +9280,10 @@ getenv_spec_function (int argc, const ch
+     value = varname;
+ 
+   if (!value)
+-    fatal_error (input_location,
+-		 "environment variable %qs not defined", varname);
++    {
++      warning (input_location, "environment variable %qs not defined", varname);
++      value = "";
++    }
+ 
+   /* We have to escape every character of the environment variable so
+      they are not interpreted as active spec characters.  A