Support GCC7 on Linux

[SeanTAllen]

Our only GCC7 issue appears to that it needs to be linked against
libatomic explicitly. This commit links to libamotic explicitly on any
Linux platform regardless of compiler version.

Closes #1756

[SeanTAllen]

This passes CI so, no problem for using with GCC6 on Trusty.

@lisael @russel could you verify this fixes #1756 for you?

[SeanTAllen]

I set up a Sid VM and verified both the issue and the fix. I'll have a different PR for instructions for building on Sid as I had to figure that out.

However there is then a problem with building pony programs.

Writing ./helloworld.o
Linking ./helloworld
Warning: environment variable $CC undefined, using cc as the linker
/home/vagrant/ponyc/build/release//libponyrt-pic.a(pool.o):pool.c:function pool_get.constprop.5: error: undefined reference to '__atomic_compare_exchange_16'
/home/vagrant/ponyc/build/release//libponyrt-pic.a(pool.o):pool.c:function ponyint_pool_alloc: error: undefined reference to '__atomic_compare_exchange_16'
/home/vagrant/ponyc/build/release//libponyrt-pic.a(pool.o):pool.c:function ponyint_pool_free: error: undefined reference to '__atomic_compare_exchange_16'
/home/vagrant/ponyc/build/release//libponyrt-pic.a(pool.o):pool.c:function ponyint_pool_thread_cleanup: error: undefined reference to '__atomic_compare_exchange_16'

[SeanTAllen]

-latomic needs to be added during the linking phase as well

[SeanTAllen]

Ok, I have fix for linux. verifying it passes our CI.

Going to check to see if fixes freebsd issue as well.

[SeanTAllen]

This doesnt work on FreeBSD. I've updated the PR to be linux only. If this passes CI, its good to merge.

[russel]

Reported in on #1756 that 499a1df seems to work fine on Debian Sid with GCC 7.