x86/brk: make the brk reservation symbols inaccessible from C

Impact: bulletproofing, clarification

The brk reservation symbols are just there to document the amount
of space reserved by brk users in the final vmlinux file.  Their
addresses are irrelevent, and using their addresses will cause
certain havok.  Name them ".brk.NAME", which is a valid asm symbol
but C can't reference it; it also highlights their special
role in the symbol table.

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