binutils, glibc: avoid DT_TEXTREL in ld.so

ld.so was using a non-PIC reloc to get the link-time address of _DYNAMIC,
which, while correct, causes the library to be flagged DT_TEXTREL.  Avoid
this by leveraging the fact that the static linker has put the link-time
address of _DYNAMIC in the first GOT entry.
binutils, gcc: make DWARF sections read-only

Use PC-relative expressions to avoid dynamic relocs in PIC.
binutils: keep .dynamic section out of text segment

This obviates writable text/executable data.