Configure for csky gcc

Basic linux gcc configuration:
  ./configure [HOST_TARGET] --enable-languages=c --target=[TARGET] --with-cskyabi=[ABI VERSION] --without-headers --with-newlib --enable-shared --disable-libssp --disable-libgomp --disable-libmudflap --with-sysroot=[SYSROOT DIRECTORY]  --prefix=[PREFIX]
HOST_TARGET is the host to execute binutils, such as x86_64-linux.
TARGET can choose one of csky-abiv2-linux,csky-linux. 
ABI VERSION is abiv1 or abiv2.
If you want to build no multilib gcc, you should add:
  --disable-multilib --with-cpu=[CPU] --with-endian=[ENDIAN] --with-float=[FLOAT VERSION]
CPU is one of csky cpu models, such as ck810f.
ENDIAN is big or little.
FLOAT VERSION is hard or soft.
If c++ is needed, build the libc fisrt, and add following to configuration.
  --enable-languages=c,c++ --enable-threads=posix --with-lib=[LIBC DIRECTORY] --with-headers=[LIBC HEADER DIRECTORY] --with-build-sysroot=[SYSROOT DIRECTORY] --enable-poison-system-directories --with-build-time-tools=[INSTALL DIRECTORY/bin]

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