This adds support for ".attribute arch" for all extensions that are
currently supported by the compiler.
Differential Revision: https://reviews.llvm.org/D94931
Leverage ARM ELF build attribute section to create ELF attribute section
for RISC-V. Extract the common part of parsing logic for this section
into ELFAttributeParser.[cpp|h] and ELFAttributes.[cpp|h].
Differential Revision: https://reviews.llvm.org/D74023