gas: further limit .rept count

gas currently limits a rept count at 0x7fffffff.  With a minimal rept
body this requires at least 2G * 16 due to the .linefile directive
added by buffer_and_nest.  I'm inclined to think 32G is excessive.

This patch limits the total memory used by rept to 4G (2G on 32-bit
systems).  That of course reduces allowed repeat counts by a factor of
at least 8, and note that the file name affects the max repeat.

The patch also changes the repeat count to 1 rather than 0 when
we hit the limit, so that the body of the rept is not entirely ignored.

Nested rept can still easily cause OOM of course.

	* read.c (do_repeat): Limit allowed repeat count based on
	total memory needed.