Add checks for `no-pie` flag for cross-compilers

The check to see if a compiler supports the `no-pie` flag was done only
for the main C compiler, and not for the cross-compilers used for
creating the executables for the different supported architectures.

This commit introduces the aforementioned missing checks.

In addition instead of hard-coding the flags to check in the CMakeLists
file we have a list that we pass each time we instantiate a project for
the cross-compilers, and we check for the availability of all the flags.

In order to do this we need to apply a sort of serialization and
deserialization to avoid the "unpack" of the list passed as argument to
the external project (that is implemented as a `;` separated string).

Also implemented a fix suggested in the merge request for a line that
mistakenly added the `TEST_CFLAGS` variable to the `NO_PIE` variable.