Makefile.system: adjust mipsel/mips64el ARCH variables
Makefile.system: adjust mipsel/mips64el ARCH variables

When building for MIPS{64} little-endian variants, the included makefiles
should be the same as for the big-endian.

There are already some adjustments being done for some ARCH names.
This change adds the ones for the `mipsel` and `mips64el` names, so that
the Makefile.mips{64} files get included.

This comes as a result of: https://github.com/openwrt/packages/issues/16649

Signed-off-by: Alexandru Ardelean <4d01ecd25c16b1b5a0ed7395b7943a3900a20b4c@gmail.com>

xianyi/OpenBLAS,xianyi/OpenBLAS,xianyi/OpenBLAS,xianyi/OpenBLAS,xianyi/OpenBLAS,xianyi/OpenBLAS,xianyi/OpenBLAS,xianyi/OpenBLAS