kselftest/arm64/mte: Introduce MTE safe memory accessors

The MTE selftests assume that they can skip (expected) MTE faults by
advancing the PC by 4 in mte_default_handler(), but this is not
generally safe, as the faults are triggered from arbitrary library
functions (e.g. memset() and memcpy()). For instance, memset() could
be implemented as simple as:

  mov	x3, x0       // copy pointer
  add	x4, x0, x2   // calculate end
loop:
  strb	w1, [x3], #1 // faulting access
  cmp	x3, x4
  b.ne	loop
  ret

which leads to an infinite loop since X3 could not be updated.

Or, it could be having advanced implementation (FEAT_MOPS):

  mov	x3, x0         // copy pointer
  setp	[x3]!, x2!, x1 // prologue
  setm	[x3]!, x2!, x1 // main
  sete	[x3]!, x2!, x1 // epilogue
  ret

with faulting access at prologue advancing PC to main could lead to
infinite loop due to main could be triggering unaligned access fault
which kernel fixing up by advancing PC back to prologue.

Instead of relying on arbitrary implementations of library functions
introduce helpers to perform the faulting accesses, where those faults
can safely be handled.

Signed-off-by: Vladimir Murzin <vladimir.murzin@arm.com>
Tested-by: Muhammad Usama Anjum <usama.anjum@arm.com>
Reviewed-by: Muhammad Usama Anjum <usama.anjum@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

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