Use the LLD linker for all but SGX libraries for 5x faster link times

[eigenraven]

From a short test on my PC:

(faasm) root@341d8f1cc516:/usr/local/code/faasm# rm /build/faasm/bin/* ; sync ; time inv dev.cc all
ld (default):

real    0m5.037s
user    0m33.545s
sys     0m11.528s

lld:

real    0m1.183s
user    0m7.354s
sys     0m5.349s

When iterating on changes, linker usually took most of the build time on my machine, this helps the link times massively. The PR enables lld on executables via the CMake configuration, and on faasm shared libraries, but not manually created libraries as flags used for building the SGX trusted enclave library were not compatible - this way that specific library is still compiled with the default ld.bfd linker.

[Shillaker]

🙏 amazing. Could we do this in Faabric too?