mips: enable CONFIG_USE_PRIVATE_LIBGCC by default

mips: enable CONFIG_USE_PRIVATE_LIBGCC by default

Without the private libgcc, we need a full multilib toolchain with
different libgcc or multiple toolchains to build all BE/LE and
hard-float/soft-float variants of MIPS boards.  That is not feasible.

This commit allows us to build all the MIPS boards with a single
kernel.org toolchain:

https://www.kernel.org/pub/tools/crosstool/files/bin/x86_64/4.9.0/
x86_64-gcc-4.9.0-nolibc_mips-linux.tar.xz

This change sounds reasonable for most users.  If necessary,
you can disable this option via "make menuconfig" or friends.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>