Remove dependency on libatomic on Linux
Due to some unfortunate interplay between clang and libstdc++, clang was
not able to correctly identify to alignment of PoolRange and
SideTableRefCountBits, causing it to emit library calls instead of
inlining atomic operations. This was fixed by adding the appropriate
alignment to those types. In addition to that the march for the Linux
target was set to 'core2', which is the earliest architecture to support
cx16, which is necessary for the atomic operations on PoolRange.
