arm64: mm: fold alternatives into .init

Currently we treat the alternatives separately from other data that's
only used during initialisation, using separate .altinstructions and
.altinstr_replacement linker sections. These are freed for general
allocation separately from .init*. This is problematic as:

* We do not remove execute permissions, as we do for .init, leaving the
  memory executable.

* We pad between them, making the kernel Image bianry up to PAGE_SIZE
  bytes larger than necessary.

This patch moves the two sections into the contiguous region used for
.init*. This saves some memory, ensures that we remove execute
permissions, and allows us to remove some code made redundant by this
reorganisation.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Andre Przywara <andre.przywara@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Jeremy Linton <jeremy.linton@arm.com>
Cc: Laura Abbott <labbott@fedoraproject.org>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>