[ELF] Emit DT_TEXTREL dynamic table flag.

If one or more dynamic relocation might modify a read-only section,
dynamic table should contain DT_TEXTREL tag.

The patch introduces new `RelocationTable::canModifyReadonlySection()`
method. This method checks through the relocations to see if any modifies
a read-only section. The DynamicTable class calls this method and emits
the DT_TEXTREL tag if necessary.

The patch reviewed by Rui Ueyama and Shankar Easwaran.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@208670 91177308-0d34-0410-b5e6-96231b3b80d8