kselftest/arm64: Check GCR_EL1 after context switch

This test is specific to MTE and verifies that the GCR_EL1 register
is context switched correctly.

It spawn 1024 processes and each process spawns 5 threads. Each thread
writes a random setting of GCR_EL1 through the prctl() system call and
reads it back verifying that it is the same. If the values are not the
same it reports a failure.

Note: The test has been extended to verify that even SYNC and ASYNC mode
setting is preserved correctly over context switching.

Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
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Change-Id: Ia917684a2b8e5f29e705ca5cbf360b010df6f61e