ARM: 8154/1: use _install_special_mapping for sigpage

ARM: 8154/1: use _install_special_mapping for sigpage

_install_special_mapping allows the VMA to be identifed in
/proc/pid/maps without the use of arch_vma_name, providing a
slight net reduction in object size:

  text    data     bss     dec     hex filename
  2996      96     144    3236     ca4 arch/arm/kernel/process.o (before)
  2956     104     144    3204     c84 arch/arm/kernel/process.o (after)

Signed-off-by: Nathan Lynch <[email protected]>
Reviewed-by: Kees Cook <[email protected]>
Signed-off-by: Russell King <[email protected]>