toolchain/gcc: add GCC 8.1.0

Changes compared to GCC 7.x

001-revert_register_mode_search.patch dropped

The underlying issue is described at the end of
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58139

It is fixed by the upstream commit:
3fa2798aa8

020-PR-libstdc-81797-Add-.NOTPARALLEL-to-include-Makefil.patch dropped due to already upstream

100-PR-rtl-optimization-83496.patch dropped due to already upstream

910-mbsd_multi.patch
modified to fix ambiguous overloaded inform() call error

gcc/input.h
header: define UNKNOWN_LOCATION ((source_location) 0)

-        inform (0, "someone does not honour COPTS correctly, passed %d times",
-         honour_copts);
+        inform (UNKNOWN_LOCATION, "someone does not honour COPTS correctly, passed %d times",
+         honour_copts);

940-no-clobber-stamp-bits.patch dropped due to fixed upstream by another way
upstream commit: 87b2d547f8

950-cpp_file_path_translation.patch dropped, Both -fmacro-prefix-map and -ffile-prefix-map are added
to gcc 8.1.0, if I understand it correctly, we should use -fmacro-prefix-map
usage: -fmacro-prefix-map=@var{old}=@var{new}
upstream commit: 859b51f836

-iremap exists as a flag for a long time, for backward compatibility, I think we should keep the
variable name unchanged but change its value in rules.mk for gcc 8.x and higher.

Compile and run tested on x86_64

Signed-off-by: Syrone Wong <wong.syrone@gmail.com>

toolchain/gcc/patches/8.1.0/010-documentation.patch

@@ -0,0 +1,35 @@
+commit 098bd91f5eae625c7d2ee621e10930fc4434e5e2
+Author: Luka Perkov <luka@openwrt.org>
+Date:   Tue Feb 26 16:16:33 2013 +0000
+
+    gcc: don't build documentation
+    
+    This closes #13039.
+    
+    Signed-off-by: Luka Perkov <luka@openwrt.org>
+    
+    SVN-Revision: 35807
+
+--- a/gcc/Makefile.in
++++ b/gcc/Makefile.in
+@@ -3203,18 +3203,10 @@ doc/gcc.info: $(TEXI_GCC_FILES)
+ doc/gccint.info: $(TEXI_GCCINT_FILES)
+ doc/cppinternals.info: $(TEXI_CPPINT_FILES)
+ 
+-doc/%.info: %.texi
+-	if [ x$(BUILD_INFO) = xinfo ]; then \
+-		$(MAKEINFO) $(MAKEINFOFLAGS) -I . -I $(gcc_docdir) \
+-			-I $(gcc_docdir)/include -o $@ $<; \
+-	fi
++doc/%.info:
+ 
+ # Duplicate entry to handle renaming of gccinstall.info
+-doc/gccinstall.info: $(TEXI_GCCINSTALL_FILES)
+-	if [ x$(BUILD_INFO) = xinfo ]; then \
+-		$(MAKEINFO) $(MAKEINFOFLAGS) -I $(gcc_docdir) \
+-			-I $(gcc_docdir)/include -o $@ $<; \
+-	fi
++doc/gccinstall.info:
+ 
+ doc/cpp.dvi: $(TEXI_CPP_FILES)
+ doc/gcc.dvi: $(TEXI_GCC_FILES)