Fix INTERFACE64 builds on Loongarch64 with LLVM

[azuresky01]

fix #5331

Compilation of OpenBLAS on loongarch64 is now possible with LLVM clang/flang(flang-new) version 20 and above. Recently I tried to build OpenBLAS 0.3.30 (should also apply to current develop version) on Debian sid with different parameters and found gmake compilation failure when INTERFACE64=1 is set.

During the compilation I found that INTERFACE64=1 is not implemented by flang and finally I got the following error message:

...
/usr/local/llvm-20.1.7/bin/flang -O2 -march=loongarch64   -o zblat2 zblat2.o ../libopenblas_la464p-r0.3.30.a -lm -lpthread -lm -lpthread -L/usr/local/llvm-20.1.7/bin/../lib/loongarch64-unknown-linux-gnu -L/usr/local/llvm-20.1.7/lib/clang/20/lib/loongarch64-unknown-linux-gnu -L/usr/lib/gcc/loongarch64-linux-gnu/14 -L/usr/lib/gcc/loongarch64-linux-gnu/14/../../../../lib64 -L/lib/loongarch64-linux-gnu -L/lib/../lib64 -L/usr/lib/loongarch64-linux-gnu -L/usr/lib/../lib64 -L/lib -L/usr/lib  -lc 
OPENBLAS_NUM_THREADS=1 OMP_NUM_THREADS=1 ./sblat1
 Real BLAS Test Program Results


 Test of subprogram number  1             SDOT 
Bus error
make[1]: *** [Makefile:51: level1] Error 135
...

To fix the above error, I added a few lines in Makefile.system to specify the support of INTERFACE64. After these changes the compilation error disappears.

[XiWeiGu]

LGTM, Thanks.