[ELF] Implement --rosegment

By default linker would not create a separate segment to hold read only data.

This option overrides that behavior by creating the a separate read only segment
for read only data.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@217358 91177308-0d34-0410-b5e6-96231b3b80d8