toolchain/gcc: Backport patch to fix unconditional MULTIARCH_DIRNAME

toolchain/gcc: Backport patch to fix unconditional MULTIARCH_DIRNAME

This backports the patch for GCC PR target/89587 (gcc's rs6000
configuration unconditionally sets MULTIARCH_DIRNAME, even when
multiarch is disabled).

This currently affects apm821xx and may cause issues when
cross-compiling packages, e.g. Python 3[1].

This includes patches for GCC 8 (with the changelog diff removed);
this change is already included in GCC 9.2 and 7.5.

[1]: https://github.com/openwrt/packages/issues/10552

Signed-off-by: Jeffery To <jeffery.to@gmail.com>
[Removed patch for GCC 7.4.0, GCC 7.5.0 already contains this]
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>