lto not working with clang

Adding --enable-ipo to the configuration line with clang or aocc breaks. The configuration scripts chokes on the -lm test. Ignoring this, the compilation of SWIFT itself fails.

On cosma, this breaks:

[login7a] > module list
Currently Loaded Modulefiles:
 1) aocc/2.2.0   2) gsl/2.4(default)  
[login7a] > ./configure CC=clang --with-hdf5=/cosma/local/hdf5//intel_2020-update2/1.10.6/bin/h5cc --disable-mpi --enable-compiler-warning --enable-ipo

The error reported in config.log is:

/opt/rh/devtoolset-3/root/usr/lib/gcc/x86_64-redhat-linux/4.9.2/../../../../bin/ld.gold: error: /cosma/local/aocc/aocc-compiler-2.2.0/bin/../lib/LLVMgold.so: could not load plugin library: /cosma/local/aocc/aoc\
c-compiler-2.2.0/bin/../lib/LLVMgold.so: cannot open shared object file: No such file or directory
/opt/rh/devtoolset-3/root/usr/lib/gcc/x86_64-redhat-linux/4.9.2/../../../../bin/ld.gold: error: /tmp/conftest-5f9d8a.o:1:3: invalid character
/opt/rh/devtoolset-3/root/usr/lib/gcc/x86_64-redhat-linux/4.9.2/../../../../bin/ld.gold: error: /tmp/conftest-5f9d8a.o:1:3: syntax error, unexpected $end
/opt/rh/devtoolset-3/root/usr/lib/gcc/x86_64-redhat-linux/4.9.2/../../../../bin/ld.gold: error: /tmp/conftest-5f9d8a.o: not an object or archive

so it seems related to the linker being used.