[ELF] - Treat .openbsd.randomdata as relro section

[grimar]

Summary

That was requested by Mark Kettenis in llvm-dev:

"It is the intention that .openbsd.randomdata sections are made
read-only after initialization. The native (ld.bfd based) OpenBSD
toolchain accomplishes this by including .openbsd.randomdata into the
PT_GNU_RELRO segment."

He suggested diff code change, I added testcase.

lld/trunk/ELF/Writer.cpp

template <class ELFT> bool elf::isRelroSection(const OutputSectionBase *Sec) {
   if (Sec == In<ELFT>::Dynamic->OutSec)
     return true;
   if (In<ELFT>::Got && Sec == In<ELFT>::Got->OutSec)
     return true;
   if (In<ELFT>::MipsGot && Sec == In<ELFT>::MipsGot->OutSec)
     return true;
   StringRef S = Sec->getName();
   return S == ".data.rel.ro" || S == ".ctors" || S == ".dtors" || S == ".jcr" ||
-         S == ".eh_frame";
+         S == ".eh_frame" || S == ".openbsd.randomdata";
 }
 
 template <class ELFT>
 static bool compareSectionsNonScript(const OutputSectionBase *A,
                                      const OutputSectionBase *B) {
   // Put .interp first because some loaders want to see that section
   // on the first page of the executable file when loaded into memory.
   bool AIsInterp = A->getName() == ".interp";

lld/trunk/test/ELF/relro.s

 // CHECK-NEXT: PHDR
 // CHECK-NEXT: LOAD
 // CHECK-NEXT: LOAD
 // CHECK-NEXT: LOAD
 // CHECK-NEXT: DYNAMIC
 // CHECK-NEXT: GNU_RELRO
 // CHECK: Section to Segment mapping:
 
-// FULLRELRO:  05     .dynamic .got .got.plt {{$}}
-// PARTRELRO:  05     .dynamic .got {{$}}
+// FULLRELRO:  05     .openbsd.randomdata .dynamic .got .got.plt {{$}}
+// PARTRELRO:  05     .openbsd.randomdata .dynamic .got {{$}}
 
 
 // NORELRO-NOT: GNU_RELRO
 
 .global _start
 _start:
   .long bar
   jmp *bar2@GOTPCREL(%rip)
 
 .section .data,"aw"
 .quad 0
 
 .zero 4
 .section .foo,"aw"
 .section .bss,"",@nobits
+
+.section .openbsd.randomdata, "aw"
+.quad 0