[lld][ELF]Add option to make .dynamic read only

The .dynamic section of an ELF almost doesn't need to be written to with
the exception of the DT_DEBUG entry. For several reasons having a read
only .dynamic section would be useful. This change adds the -z keyword
"rodynamic" which forces .dynamic to be read-only. In this case DT_DEBUG
will not be emited.

Patch by Jake Ehrlich

Differential Revision: https://reviews.llvm.org/D33251

llvm-svn: 304024