python3: RDEPEND on libgcc

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Python uses features of glibc that require it to dynamically load (i.e.
dlopen()) libgcc_s at runtime. However, since this isn't a link time
dependency, it doesn't get picked up automatically by bitbake so
manually add it to RDEPENDS.

There is an outstanding bug in Python to make it explicitly link against
libgcc at link time which would remove the need for this. See:
https://bugs.python.org/issue37395



Signed-off-by: Joshua Watt <JPEWhacker@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>