Support larger guard regions.

This also fixes a long-standing bug where the guard region would be taken
out of the stack itself, rather than being -- as POSIX demands -- additional
space after the stack. Historically a 128KiB stack with a 256KiB guard would
have given you an immediate crash.

Bug: http://b/38413813
Test: builds, boots
Change-Id: Idd12a3899be1d92fea3d3e0fa6882ca2216bd79c