gcc: remove paths to sysroot from configargs.h and checksum-options for gcc-cross...

gcc: remove paths to sysroot from configargs.h and checksum-options for gcc-cross-canadian

Apply fixes from gcc-cross (84a78f46d594 and 0ead8cbdfb96) to gcc-cross-canadian.
This will improve (but not fix) reproducibility of gcc-cross-canadian.
Also move this code to functions to avoid code duplication.

[RP: Tweak patch to make the function parameters clear and fix quoting issues
ensuring the code exactly matches the original replacements with an additional
parameter.]

Signed-off-by: Oleksandr Hnatiuk <ohnatiuk@cisco.com>
Signed-off-by: Denys Dmytriyenko <denys@konsulko.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>