Add new uvm function called uvm_map_pie() which takes align as a

Add new uvm function called uvm_map_pie() which takes align as a
parameter and returns an aligned random load address for position
independent executables to use. This also adds three new vmparam.h
defines to specify the maximum address, minimum address and minimum
allowed alignment for uvm_map_pie() to use. The PIE address range
for i386 was carefully selected to work well within the i386 W^X
framework.

With much help and feedback from weingart@.
okay weingart@, miod@, kettenis@, drahn@