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: NMasahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: NDaniel Schwierzeck <daniel.schwierzeck@gmail.com>

arch/mips/Kconfig

@@ -4,6 +4,9 @@ menu "MIPS architecture"
 config SYS_ARCH
 	default "mips"
 
+config USE_PRIVATE_LIBGCC
+	default y
+
 choice
 	prompt "Target select"