MIPS: pic32mzdask: use CONFIG_USE_PRIVATE_LIBGCC=y

MIPS: pic32mzdask: use CONFIG_USE_PRIVATE_LIBGCC=y

MIPS EL boards should define CONFIG_USE_PRIVATE_LIBGCC=y to work
with EB-only toolchains like the one from kernel.org. If one do
not globally set CONFIG_USE_PRIVATE_LIBGCC=y, the build fails with:

/opt/gcc-4.9.0-nolibc/mips-linux/bin/mips-linux-ld.bfd: /opt/gcc-4.9.0-nolibc/mips-linux/bin/../lib/gcc/mips-linux/4.9.0/libgcc.a(_lshrdi3.o): compiled for a big endian system and target is little endian
/opt/gcc-4.9.0-nolibc/mips-linux/bin/mips-linux-ld.bfd: /opt/gcc-4.9.0-nolibc/mips-linux/bin/../lib/gcc/mips-linux/4.9.0/libgcc.a(_lshrdi3.o): endianness incompatible with that of the selected emulation
/opt/gcc-4.9.0-nolibc/mips-linux/bin/mips-linux-ld.bfd: failed to merge target specific data of file /opt/gcc-4.9.0-nolibc/mips-linux/bin/../lib/gcc/mips-linux/4.9.0/libgcc.a(_lshrdi3.o)
/opt/gcc-4.9.0-nolibc/mips-linux/bin/mips-linux-ld.bfd: /opt/gcc-4.9.0-nolibc/mips-linux/bin/../lib/gcc/mips-linux/4.9.0/libgcc.a(_ashldi3.o): compiled for a big endian system and target is little endian
/opt/gcc-4.9.0-nolibc/mips-linux/bin/mips-linux-ld.bfd: /opt/gcc-4.9.0-nolibc/mips-linux/bin/../lib/gcc/mips-linux/4.9.0/libgcc.a(_ashldi3.o): endianness incompatible with that of the selected emulation
/opt/gcc-4.9.0-nolibc/mips-linux/bin/mips-linux-ld.bfd: failed to merge target specific data of file /opt/gcc-4.9.0-nolibc/mips-linux/bin/../lib/gcc/mips-linux/4.9.0/libgcc.a(_ashldi3.o)
/work/git-trees/u-boot-mips/Makefile:1171: recipe for target 'u-boot' failed

One example for a failing build is Travis CI.

Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Reviewed-by: Purna Chandra Mandal <purna.mandal@microchip.com>