[RuntimeDyld] Fix building on OpenBSD

[brad]

Summary

With https://reviews.llvm.org/D105466 the tree does not build on OpenBSD/amd64. Moritz suggested only building this code on Linux.

[MoritzS]

The test failures all seem to be unrelated to this change, lgtm.

llvm/tools/llvm-rtdyld/llvm-rtdyld.cpp

 
 private:
   std::map<std::string, uint64_t> DummyExterns;
   sys::MemoryBlock PreallocSlab;
   bool UsePreallocation = false;
   uintptr_t SlabSize = 0;
   uintptr_t CurrentSlabOffset = 0;
   SectionIDMap *SecIDMap = nullptr;
-#if defined(__x86_64__) && defined(__ELF__)
+#if defined(__x86_64__) && defined(__ELF__) && defined(__linux__)
   unsigned UsedTLSStorage = 0;
 #endif
 };
 
 uint8_t *TrivialMemoryManager::allocateCodeSection(uintptr_t Size,
                                                    unsigned Alignment,
                                                    unsigned SectionID,
                                                    StringRef SectionName) {
if (!MB.base())
     report_fatal_error(Twine("MemoryManager allocation failed: ") +
                        EC.message());
   DataMemory.push_back(SectionInfo(SectionName, MB, SectionID));
   return (uint8_t*)MB.base();
 }
 
 // In case the execution needs TLS storage, we define a very small TLS memory
 // area here that will be used in allocateTLSSection().
-#if defined(__x86_64__) && defined(__ELF__)
+#if defined(__x86_64__) && defined(__ELF__) && defined(__linux__)
 extern "C" {
 alignas(16) __attribute__((visibility("hidden"), tls_model("initial-exec"),
                            used)) thread_local char LLVMRTDyldTLSSpace[16];
 }
 #endif
 
 TrivialMemoryManager::TLSSection
 TrivialMemoryManager::allocateTLSSection(uintptr_t Size, unsigned Alignment,
                                          unsigned SectionID,
                                          StringRef SectionName) {
-#if defined(__x86_64__) && defined(__ELF__)
+#if defined(__x86_64__) && defined(__ELF__) && defined(__linux__)
   if (Size + UsedTLSStorage > sizeof(LLVMRTDyldTLSSpace)) {
     return {};
   }
 
   // Get the offset of the TLSSpace in the TLS block by using a tpoff
   // relocation here.
   int64_t TLSOffset;
   asm("leaq LLVMRTDyldTLSSpace@tpoff, %0" : "=r"(TLSOffset));