x86, mm: Add RESERVE_BRK_ARRAY() helper

This is useful when converting static arrays into boot-time brk
allocated objects.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
LKML-Reference: <4C805EEA.1080205@goop.org>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>