python3: fixing bpo-36852 patch for builds on i686 architecture

The original patch "bpo-36852: proper detection of mips architecture
for soft float" uses AC_CANONICAL_TARGET to determine the platform
triplet. While AC_CANONICAL_TARGET exports i686 as target_cpu, gcc
is using i386 instead. We fall back here to i386, as it is conform
to the previous behavior.

Upstream Status: Submitted [https://github.com/python/cpython/pull/13196

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Signed-off-by: Matthias Schoepfer <matthias.schoepfer@ithinx.io>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>