[RuntimeDyld] Fix building on OpenBSD

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

Reviewed By: MoritzS

Differential Revision: https://reviews.llvm.org/D119991

Author: brad0

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

@@ -286,7 +286,7 @@ class TrivialMemoryManager : public RTDyldMemoryManager {
   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
 };
@@ -350,7 +350,7 @@ uint8_t *TrivialMemoryManager::allocateDataSection(uintptr_t Size,
 
 // 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];
@@ -361,7 +361,7 @@ 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 {};
   }