[ELF] Add ELFFileBase::{elfShdrs,numELFShdrs} to avoid duplicate llvm::object::ELFFil...

[ELF] Add ELFFileBase::{elfShdrs,numELFShdrs} to avoid duplicate llvm::object::ELFFile::sections()

This mainly avoid `relsOrRelas` cost in `InputSectionBase::relocate`.
`llvm::object::ELFFile::sections()` has redundant and expensive checks.