Fix building with GCC 8+

[SeanTAllen]

Starting with GCC 8, they added a new warning stringops-truncation. It
is well intentioned but, seems prone to a lot of false positives.

It found several false positives in the Pony codebase and because we treat
all warnings as errors, you can't build ponyc with GCC 8+.

This commit switches from using strncpy to memcpy. They are in the end, the
same operation except a difference in return type. However, with the new
gcc option, memcpy will not complain about not copying the null terminator.

Fixes #3313

[SeanTAllen]

For reasons that I don't understand, I was unable to reproduce #3313 with GCC 8.3 on my Ubuntu install. I've asked @stronny to verify that this fixes the issue for them. This is good for review, but I've put a "DO NOT MERGE" on until we get some independent verification it fixes the issue.

[dipinhora]

reproduces error:

docker run --rm -it fedora sh -c 'dnf install -y git gcc-c++ make zlib-devel llvm3.9-devel ncurses-devel  libatomic which findutils && git clone https://github.com/ponylang/ponyc && cd ponyc && LLVM_CONFIG=/usr/lib64/llvm3.9/bin/llvm-config make -j`nproc`'

confirms fix:

docker run --rm -it fedora sh -c 'dnf install -y git gcc-c++ make zlib-devel llvm3.9-devel ncurses-devel  libatomic which findutils && git clone https://github.com/ponylang/ponyc && cd ponyc && git checkout 3313 && LLVM_CONFIG=/usr/lib64/llvm3.9/bin/llvm-config make -j`nproc`'

[SeanTAllen]

Excellent. Thanks, @dipinhora.