Generic ABI as of 2000  mentioned that "Its [DT_TEXTREL's] use
has been superseded by the DF_TEXTREL flag". We currently create only
DT_TEXTREL and don't set the DF_TEXTREL flag.
This patch is to let the linker to set the flag instead of creating
a DT_TEXTREL marker segment.
With this patch, lld no longer emits DT_TEXTREL. I believe that's not
an issue because it is a very old option and text relocation is rare.
If you want to keep it, we can set both DT_TEXTREL and DF_TEXTREL though.
But I'm not comfortable with supporting only the old way.