C-SKY: Fix wrong sysroot suffix when disable multilib.

C-SKY: Fix wrong sysroot suffix when disable multilib.

The SYSROOT_SUFFIX_SPEC works even when multilib is disabled.
So when build no-multilib glibc toolchain and the options are
not same as MULTILIB_DEFAULTS, the sysroot will specify wrong
because the libc will not be installed as such.
This bug causes glibc regression test error:
  https://sourceware.org/pipermail/libc-testresults/2023q1/010706.html
The error is:
  /scratch/jmyers/glibc-bot/install/compilers/csky-linux-gnuabiv2/csky-glibc-linux-gnuabiv2/bin/ld: cannot find -lc: No such file or directory

gcc/
	* config.gcc(csky-*-linux*): Define CSKY_ENABLE_MULTILIB
	and only include 'csky/t-csky-linux' when enable multilib.
	* config/csky/csky-linux-elf.h(SYSROOT_SUFFIX_SPEC): Don't
	define it when disable multilib.