x86/brk: put the brk reservations in their own section

Impact: disambiguate real .bss variables from .brk storage

Add a .brk section after the .bss section.  This has no effect
on the final vmlinux, but it more clearly distinguishes the space
taken by actual .bss symbols, and the variable space reserved
by .brk users.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>