Canonicalize the type of .eh_frame.

GNU as can give it type SHT_PROGBITS or SHT_X86_64_UNWIND depending on
teh construct.

MC gives it type SHT_X86_64_UNWIND.

The linker has to canonicalize to one or the other so that there is only
one .eh_frame in the end.

llvm-svn: 252757