Handle .space directive with non-constant operand:

Handle .space directive with non-constant operand:

* read.c (s_space): Rewrite to handle general expressions.  Generate rs_space
frags for non-constant values.
* write.c (cvt_frag_to_fill): Treat rs_align_code and rs_space like rs_align
and rs_org.  Verify that fr_offset is non-negative, and force frag type to
rs_fill only after assertion checks.
(relax_segment): Treat rs_align_code like rs_align.  Treat rs_space like rs_org
in the first switch; in the second, force the operand to a constant, and use it
for the growth size.