Threshold at which Binary literals (e.g. strings)
are either dumped to a file and a CmmFileEmbed literal
is emitted (over threshold), or become a CmmString
Literal (under or at threshold). CmmFileEmbed is only supported
with the NCG, thus a Just means two things: We have a threshold,
and will be using the NCG. Conversely, a Nothing implies we are not
using NCG and disables CmmFileEmbed. See Note
[Embedding large binary blobs] in GHC.CmmToAsm.Ppr, and
cgTopBinding in GHC.StgToCmm.