Bug#1137143: llvm-toolchain-21: flang-21 fails to link on ppc64el

[Alastair McKinstry]

Source: llvm-toolchain-21
Version: 1:21.1.8-7
Severity: normal

When trying to link fortran-assert, I get this on ppc64el:
```
[ 22%]                   libassert.so
 + flang-21 -shared   -Wl,-z,relro -L/usr/lib/powerpc64le-linux-gnu/fortran/flang build/flang-21_3A97A147075E76D7/assert/src_assert_m.F90.o -Lbuild/flang-21_2C869400E79A318E    -o build/flang-21_2C869400E79A318E/libassert.so
/usr/bin/powerpc64le-linux-gnu-ld.bfd: 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)
[ 33%]                   libassert.so  done.
```

Thanks
Alastair McKinstry



-- System Information:
Debian Release: 13.5
  APT prefers stable
  APT policy: (500, 'stable')
Architecture: amd64 (x86_64)
Foreign Architectures: i386

Kernel: Linux 6.12.86+deb13-amd64 (SMP w/4 CPU threads; PREEMPT)
Locale: LANG=en_IE.UTF-8, LC_CTYPE=en_IE.UTF-8 (charmap=UTF-8) (ignored: LC_ALL set to en_IE.UTF-8), LANGUAGE=en_IE:en
Shell: /bin/sh linked to /usr/bin/dash
Init: systemd (via /run/systemd/system)
LSM: AppArmor: enabled