LoongArch: Update ELF e_flags handling according to specification.

  Update handling of e_flags according to the documentation
  update [1] (discussions [2][3]).

  Object file bitness is now represented in the EI_CLASS byte.
  The e_flags field is now interpreted as follows:

  e_flags[2:0]: Base ABI modifier

  - 0x1: soft-float
  - 0x2: single-precision hard-float
  - 0x3: double-precision hard-float

  e_flags[7:6]: ELF object ABI version

  - 0x0: v0
  - 0x1: v1

  [1]: https://github.com/loongson/LoongArch-Documentation/blob/main/docs/LoongArch-ELF-ABI-EN.adoc#e_flags-identifies-abi-type-and-version
  [2]: https://github.com/loongson/LoongArch-Documentation/pull/61
  [3]: https://github.com/loongson/LoongArch-Documentation/pull/47