sim-se: Avoid function overloading for syscall implementation

sim-se: Avoid function overloading for syscall implementation

This patch is aligning the readlink and access syscalls to the open one,
which is not overloading the openFunc, but it is factoring the
implementation into a openImpl, which is used by both open and openat.

This is needed if passing them to std::function, whose constructor is
not able to handle overloaded functions.

Change-Id: I50a8aacdfd675181b6fe9a2696220ee29cc5bc4b
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/23260
Reviewed-by: Brandon Potter <Brandon.Potter@amd.com>
Maintainer: Brandon Potter <Brandon.Potter@amd.com>
Tested-by: kokoro <noreply+kokoro@google.com>