toolchain/gcc: add GCC 8.1.0

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:
https://github.com/gcc-mirror/gcc/commit/3fa2798aa887d141d86985240f03e2f3809e7e62

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: https://github.com/gcc-mirror/gcc/commit/87b2d547f8ac9778d66909b8726fe967d1efbc74

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: https://github.com/gcc-mirror/gcc/commit/859b51f83662d01e4f158ea9327db9ca37fe70b3

-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>