ELF: Define mergeHeaderFlags to ELFLinkingContext.

Only MIPS defined the member function, but this feature is not actually
MIPS-specific. Also, the dependency to the MIPS-only member function
prevented us from merging <Arch>ELF{Object,DSO}Reader classes.

This patch moves the feature from MipsLinkingContext to LinkingContext.

llvm-svn: 234068