Add various defines and stubs for enabling MTE on AArch64 sysv-like
systems such as Linux. The HWCAP feature bit is copied over in the
same way as other feature bits. Similarly we add a new wrapper header
for mman.h to define the PROT_MTE flag that can be used with mmap and
related functions.
We add a new field to struct cpu_features that can be used, for
example, to check whether or not certain ifunc'd routines should be
bound to MTE-safe versions.
Finally, if we detect that MTE should be enabled (ie via the glibc
tunable); we enable MTE during startup as required.
Support in the Linux kernel was added in version 5.10.
Reviewed-by: Szabolcs Nagy <szabolcs.nagy@arm.com>
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