[Flang] Switch flang-rt build to runtimes-based build

[mjklemm]

This PR switches the build system from the in-tree build of the Fortran runtime to the new runtimes-style build.

[dpalermo]

I am also seeing a build failure that Michael worked around by setting LD_LIBRARY_PATH. Looks like comgr ASAN build can no longer locate libcalgn_rt.asan-x86_64.so:

./bc2h: error while loading shared libraries: libclang_rt.asan-x86_64.so: cannot open shared object file: No such file or directory
make[2]: *** [CMakeFiles/oclc_abi_version_400_header.dir/build.make:75: include/oclc_abi_version_400.inc] Error 127
...
 !!!  build_aomp.sh: BUILD FAILED FOR COMPONENT comgr !!!
###     status: 1, allstat: 1

[r14 /work1/omp-nightly/build/git]$ ldd ./aomp21.0/build/comgr/asan/bc2h
        linux-vdso.so.1 (0x00007fffb19dd000)
        libclang_rt.asan-x86_64.so => not found
        libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f6a3d800000)
        /lib64/ld-linux-x86-64.so.2 (0x00007f6a3dbed000)

Even though it is in the same location is was before this change:

Build with this change:

[r14 /work1/omp-nightly/build/rocm]$ ll ./aomp_21.0-1/lib/llvm/lib/clang/21/lib/linux/libclang_rt.asan-x86_64.so
3160 -rw-r--r-- 1 dpalermo dpalermo 3231896 Apr 23 14:22 ./aomp_21.0-1/lib/llvm/lib/clang/21/lib/linux/libclang_rt.asan-x86_64.so

Previous build that worked fine:

[r14 /work1/omp-nightly/build/rocm-s0]$ ll ./aomp_21.0-1/lib/llvm/lib/clang/21/lib/linux/libclang_rt.asan-x86_64.so
3160 -rw-r--r-- 1 dpalermo dpalermo 3231896 Apr 23 12:19 ./aomp_21.0-1/lib/llvm/lib/clang/21/lib/linux/libclang_rt.asan-x86_64.so

[dpalermo]

Also note that there isn't a libclang_rt dependency in comgr/bc2h, just the comgr/asan/bc2h:

[r14 /work1/omp-nightly/build/git]$ ldd aomp21.0/build/comgr/bc2h
        linux-vdso.so.1 (0x00007ffc55733000)
        libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007fb969c00000)
        /lib64/ld-linux-x86-64.so.2 (0x00007fb96a011000)

[dpalermo]

Some logic in the comgr cmake looks like it breaks when flang-rt is added to LLVM_RUNTIMES.

[mjklemm]

Hm. I'm unsure how to fix that one. Help will be greatly appreciated!

[dpalermo]

Same problem occurs even if compiler-rt is listed last (figured it was worth a try ;-)

LLVM_RUNTIMES="libcxx;libcxxabi;libunwind;openmp;offload;flang-rt;compiler-rt"