toolchain: add support for GCC 12

GCC 12.1 is out. Add support for it.

Deleted (upstreamed):
011-v12-configure-define-TARGET_LIBC_GNUSTACK-on-musl.patch
931-libffi-fix-MIPS-softfloat-build-issue.patch

Deleted (unneeded?)
970-macos_arm64-building-fix.patch

Other patches manually rebased due to C++ conversion and consequent file name
changing (.c to .cc).

Signed-off-by: Rui Salvaterra <rsalvaterra@gmail.com>

toolchain/gcc/patches/12.1.0/010-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
+@@ -3366,18 +3366,10 @@ 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)