elf: Introduce the ARM MTE ELF segment type

Memory tags will be dumped in the core file as segments with their own
type. Discussions with the binutils and the generic ABI community
settled on using new definitions in the PT_*PROC space (and to be
documented in the processor-specific ABIs).

Introduce PT_ARM_MEMTAG_MTE as (PT_LOPROC + 0x1). Not included in this
patch since there is no upstream support but the CHERI/BSD community
will also reserve:

  #define PT_ARM_MEMTAG_CHERI    (PT_LOPROC + 0x2)
  #define PT_RISCV_MEMTAG_CHERI  (PT_LOPROC + 0x3)

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Luis Machado <luis.machado@linaro.org>
Link: https://lore.kernel.org/r/20220131165456.2160675-3-catalin.marinas@arm.com
Signed-off-by: Will Deacon <will@kernel.org>