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/7.2.0/851-libgcc_no_compat.patch

@@ -0,0 +1,22 @@
+commit 64661de100da1ec1061ef3e5e400285dce115e6b
+Author: Felix Fietkau <nbd@openwrt.org>
+Date:   Sun May 10 13:16:35 2015 +0000
+
+    gcc: add some size optimization patches
+    
+    Signed-off-by: Felix Fietkau <nbd@openwrt.org>
+    
+    SVN-Revision: 45664
+
+--- a/libgcc/config/t-libunwind
++++ b/libgcc/config/t-libunwind
+@@ -2,8 +2,7 @@
+ 
+ HOST_LIBGCC2_CFLAGS += -DUSE_GAS_SYMVER
+ 
+-LIB2ADDEH = $(srcdir)/unwind-sjlj.c $(srcdir)/unwind-c.c \
+-  $(srcdir)/unwind-compat.c $(srcdir)/unwind-dw2-fde-compat.c
++LIB2ADDEH = $(srcdir)/unwind-sjlj.c $(srcdir)/unwind-c.c
+ LIB2ADDEHSTATIC = $(srcdir)/unwind-sjlj.c $(srcdir)/unwind-c.c
+ 
+ # Override the default value from t-slibgcc-elf-ver and mention -lunwind