Move __fentry__ version definition to sysdeps/{i386,x86_64}

__fentry__ symbol is currently not defined for other architectures.
Attempts to introduce it cause abicheck to fail, because it will be
available since 2.29 earliest, and not 2.13, which is the case for
Intel.  With the new code, abicheck passes for i686-linux-gnu,
x86_64-linux-gnu and x86_64-linux-gnu32 triples.

ChangeLog:

	* stdlib/Versions: Remove __fentry__.
	* sysdeps/i386/Versions: Add __fentry__.
	* sysdeps/x86_64/Versions: Add __fentry__.