fuzz: pass -Dc_args=/-Dcpp_args= to fuzzer targets

fuzz: pass -Dc_args=/-Dcpp_args= to fuzzer targets

Prompted by #29972, because right now it's practically impossible to pass
-fno-sanitize=function to the fuzzer targets without some extensive
sed'ing.

This splits both c_args and cpp_args to separate arguments for
tools/meson-build.sh, because the other way would be to use `eval`, so
the space-separated but quoted strings passed to these options are not
split where they shouldn't, and I'd rather avoid using `eval` if
possible.

Also, this switches the positional arguments we pass to `meson setup`,
as they were in incorrect order (docs say it should be buildir followed
by sourcedir); meson is apparently clever enough to figure this out and
switch the arguments around if necessary, so it didn't complain.