bitbake.conf, gcc toolchain: add -fcanon-prefix-map to map-prefixes

bitbake.conf, gcc toolchain: add -fcanon-prefix-map to map-prefixes

This patch adds -fcanon-prefix-map to the list of compile file-prefixes
list in case at least gcc 13 is used (it's a gcc-only flag).

This flag used to be part of this list in the past, but was removed with [1].

Since then the source file paths are not canonicalized, which makes
the system to miss some (and sometimes all) source files to include
in the corresponding -src packages. As an example sqlite3 and audiofile
(from meta-oe) produces empty src packages, mostly due to the pathes:
they frequently use relative paths that don't match up with the
absolute path specified in the file-preifx map:
`pwd`/../../foo.cpp and ${S}/foo.cpp might refer to the same file, but
the first one won't match the prefix-map, so it is omitted.

This patch adds this option again with gcc in the target case.

Signed-off-by: Gyorgy Sarvari <skandigraun@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>