Split the old READ_IMPLIES_EXEC workaround from executable PT_GNU_STACK

now that toolchains long support PT_GNU_STACK marking and there's no
need anymore to force modern programs into having all its user mappings
executable instead of only the stack and the PROT_EXEC ones. Disable
that automatic READ_IMPLIES_EXEC forcing on x86-64 and arm64. Add tables
documenting how READ_IMPLIES_EXEC is handled on x86-64, arm and arm64.
By Kees Cook.
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