s390/mm: align 64-bit PIE binaries to 4GB

The base address (STACK_TOP / 3 * 2) for a 64-bit program is two thirds
into the 4GB segment at 0x2aa00000000. The randomization added on z13
can eat another 1GB of the remaining 1.33GB to the next 4GB boundary.
In the worst case 300MB are left for the executable + bss which may
cross into the next 4GB segment. This is bad for branch prediction,
therefore align the base address to 4GB to give the program more room
before it crosses the 4GB boundary.

Change-Id: I45d05b33fbbbb08e4c254484749bf4b21f656b6a
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Git-commit: 4ba2815d3bf38d6a959d2d11b08cf862550dcfcc
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Se Wang (Patrick) Oh <sewango@codeaurora.org>