Bug#1117534: flang-21 cannot link due to missing libflang_rt.runtime

[Kurt Hornik]

Package: flang-21
Version: 21.1.2-3

For me flang-21 currently in unstable always fails to link as it cannot
find libflang_rt.runtime:

$ cat conftest.f 
      PROGRAM DisplayNumbers
      
      INTEGER :: i
      
      DO 99 i = 1, 10
         PRINT *, i
 99   CONTINUE

      END PROGRAM

$ flang-21 -o conftest conftest.f
/usr/bin/ld: cannot find -lflang_rt.runtime: No such file or directory
flang-21: error: linker command failed with exit code 1 (use -v to see invocation)

$ flang-21 -o conftest -v conftest.f
Debian flang version 21.1.2 (3)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/lib/llvm-21/bin
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/14
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/15
Selected GCC installation: /usr/lib/gcc/x86_64-linux-gnu/15
Candidate multilib: .;@m64
Candidate multilib: 32;@m32
Candidate multilib: x32;@mx32
Selected multilib: .;@m64
 "/usr/lib/llvm-21/bin/flang" -fc1 -triple x86_64-pc-linux-gnu -emit-obj -fcolor-diagnostics -mrelocation-model pic -pic-level 2 -pic-is-pie -target-cpu x86-64 -resource-dir /usr/lib/llvm-21/lib/clang/21 -mframe-pointer=all -o /tmp/conftest-e36de6.o -x f95 conftest.f
 "/usr/bin/ld" --hash-style=gnu --build-id --eh-frame-hdr -m elf_x86_64 -pie -dynamic-linker /lib64/ld-linux-x86-64.so.2 -o conftest /lib/x86_64-linux-gnu/Scrt1.o /lib/x86_64-linux-gnu/crti.o /usr/lib/gcc/x86_64-linux-gnu/15/crtbeginS.o -L/usr/lib/gcc/x86_64-linux-gnu/15 -L/usr/lib/gcc/x86_64-linux-gnu/15/../../../../lib64 -L/lib/x86_64-linux-gnu -L/lib/../lib64 -L/usr/lib/x86_64-linux-gnu -L/usr/lib64 -L/lib -L/usr/lib /tmp/conftest-e36de6.o -L/usr/lib/llvm-21/lib/clang/21/lib/linux -L/usr/lib/llvm-21/lib -lflang_rt.runtime -lm -lgcc --as-needed -lgcc_s --no-as-needed -lc -lgcc --as-needed -lgcc_s --no-as-needed /usr/lib/gcc/x86_64-linux-gnu/15/crtendS.o /lib/x86_64-linux-gnu/crtn.o
/usr/bin/ld: cannot find -lflang_rt.runtime: No such file or directory
flang-21: error: linker command failed with exit code 1 (use -v to see invocation)

This is with current Debian testing and the LLVM 21 compilers from
unstable.