Add size of FP environment to DataLayout
This change extends the layout specification with a new item. The item
has the form "fe:<size>" in which the argument <size> specifies the size
of floating point environment, which includes rounding mode, raised
exceptions etc.
Saving/restoring the FP environment requries creation of a variable for
the FP state but the size of this variable depends on the used target. As
this operation occurs at IR level, the size of FP state must be provided
by a component intended to represent target properties in
target-independent pipeline.