toolchain/gcc: Don't use no_sanitize("address") with gcc

GCC doesn't actually need this treatment, only clang.  (GCC is smart
enough to use "const" as the needed clue to suppress instrumentation
it seems like).  And it turns out older gcc's emit a warning
(attribute ignored, which is... sort of weird to warn about?) in that
circumstance, which breaks builds.  Just turn it off rather than try
to find a way to suppress that one warning.

Signed-off-by: Andy Ross <andyross@google.com>