[hwasan] Add fixed_shadow_base flag (#73980)

When set to non-zero, the HWASan runtime will map the shadow base at the
specified constant address.

This is particularly useful in conjunction with the existing compiler
option
'hwasan-mapping-offset', which bakes a hardcoded constant address into
the instrumentation.

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Co-authored-by: Thurston Dang <thurston@google.com>