This patch makes code less readable but it will clean itself after all functions are converted.
Differential Revision: https://reviews.llvm.org/D138665
Add support of parsing .csky_attribute directive and emit related target attributes in .csky.attribute section.
It does not emit attribute directive in assembly code, so only emit target attributes in ELF streamer.
In ELF streamer, it handles the header EFlag and the csky_attribute section which contains some attribute items.
The EFlag and attribute items are calculated from feature bits based on Subtarget.
