[LoongArch] Encode LoongArch specific ELF e_flags to binary by LoongArchTargetStreamer

[LoongArch] Encode LoongArch specific ELF e_flags to binary by LoongArchTargetStreamer

Reference: https://github.com/loongson/LoongArch-Documentation
The last commit hash (main branch) is:
99016636af64d02dee05e39974d4c1e55875c45b

Note:
There are several PRs [1][2][3] that may affect the e_flags.
After they got closed or merged, we should update the implementation here accordingly.

[1] https://github.com/loongson/LoongArch-Documentation/pull/33
[2] https://github.com/loongson/LoongArch-Documentation/pull/47
[2] https://github.com/loongson/LoongArch-Documentation/pull/61

Differential Revision: https://reviews.llvm.org/D130239