[CSKY] Lower leaf DAG node such as global symbol, frame address and jumptable, etc.

Lower global symbols such as call/external symbol.
Lower other leaf DAG node such as frame address/block address/jumptable/vastart.

Normally some leaf symbols need reside in constant pool as ABI prefers, and are addressed by
lrw or jsri instructions.

Every symbol in constant pool is lowered with one entry in target constant pool. The
entry has different type corresponding to different leaf node such as blockaddress,
jumptable, or global value.