Enable full -Wall once bugfixed GFortran 15 is released

[krystophny]

Description

An upstream GCC bug described in https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120470 and https://gcc.gnu.org/bugzilla/show_bug.cgi?id=119928 currently requires adding -Wno-external-argument-mismatch after -Wall. This should be limited to the buggy version.

Possible Solution

Once a fixed minor GCC 15 version is released, use the -Wno-external-argument-mismatch only for the previous ones.

Additional Information

#1145
#1150
fortran-lang/test-drive#49