gcc: some reword of PIE/SSP/ASLR note

Expand tabs to 8 spaces like everywhere else in the book.

Explain that shared libraries are already covered by ASLR, PIE expands
the ASLR to cover the exetutables.

In 2022, stack smashing attackings are mostly constructing a sequence of
faked returning addresses to exectute a series of function already
existing in the programs or libraries itself (ret2lib).  Returning into
the code injected by the attacker is almost impossible because on
i686 (with a PAE/NX enabled kernel) or x86_64, running injected code
needs W/X mappings and those are very rare these days.

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