[JITLink][ELF] Only make sections with SHF_WRITE writable

[jobnoorman]

Summary

All non-executable sections used to be mapped as RW- causing read-only
sections such as .rodata to be writable.

Note that this patch doesn't include a test case since it's not entirely
clear to me if it's possible to test MemProt flags.

[lhames]

LGTM. Thanks @jobnoorman!

llvm/lib/ExecutionEngine/JITLink/ELFLinkGraphBuilder.h

for (ELFSectionIndex SecIndex = 0; SecIndex != Sections.size(); ++SecIndex) {
     }
 
     LLVM_DEBUG({
       dbgs() << "    " << SecIndex << ": Creating section for \"" << *Name
              << "\"\n";
     });
 
     // Get the section's memory protection flags.
-    orc::MemProt Prot;
+    orc::MemProt Prot = orc::MemProt::Read;
     if (Sec.sh_flags & ELF::SHF_EXECINSTR)
-      Prot = orc::MemProt::Read | orc::MemProt::Exec;
-    else
-      Prot = orc::MemProt::Read | orc::MemProt::Write;
+      Prot |= orc::MemProt::Exec;
+    if (Sec.sh_flags & ELF::SHF_WRITE)
+      Prot |= orc::MemProt::Write;
 
     // Look for existing sections first.
     auto *GraphSec = G->findSectionByName(*Name);
     if (!GraphSec) {
       GraphSec = &G->createSection(*Name, Prot);
       // Non-SHF_ALLOC sections get NoAlloc memory lifetimes.
       if (!(Sec.sh_flags & ELF::SHF_ALLOC)) {
         GraphSec->setMemLifetimePolicy(orc::MemLifetimePolicy::NoAlloc);