fix Issue 11171 - text relocations in shared phobos library

- Read-only data with relocations cannot be added to CDATA,
  because text relocations are prohibited by many security
  checks (SELinux).
  Also text relocations effectively break sharing of
  read-only segments across multiple processes.
use .data.rel.ro segment for constant data with relocations

- The runtime linker will mark this as read only
  after relocations are processed.
Merge pull request #3187 from MartinNowak/fix11171

[dll] fix Issue 11171 – Text relocations in Phobos shared library