main/gcc: disable -fsplit-stack on non-glibc targets
The -fsplit-stack option requires specific fields at hardcoded offsets in the pthread_t TCB, musl does not have these fields. Therefore, enabling this compiler option on musl causes silent memory corruptions in the pthread_t TCB (on s390x it presently corrupts the cancel member of the TCB). gcc-go enables this option by default if it is available (x86 and s390x), thereby causing corruption of the TCB on these architectures. This commit fixes these TCB corruptions and thus enables bootstrapping of Google's Go implementation with gcc-go on s390x (see !27944 (merged)).