fix Issue 11171 – Text relocations in Phobos shared library

- When compiling PIC put read-only data with pointers (relocations) in
  the writeable .data.rel.ro section. The runtime linker will mark it
  as read-only after processing the relocations.

- This also fixes sharing of the read-only (TEXT) segment across
  multiple processes.

[Issue 11171](https://d.puremagic.com/issues/show_bug.cgi?id=11171 "Text relocations in Phobos shared library")