Fix cross-compiling to FreeBSD

* When testing whether we need to pass `-Wl,-z,defs` the cmake files currently
  tests for FreeBSD, but only if the host system is FreeBSD. Instead we should
  test whether the target that we're compiling is FreeBSD so we correctly deduce
  that it should not be passed.

* The `PATH_MAX` constant is defined in a different place for the dsymutil tool,
  so we just config that code out for now as it apparently doesn't include the
  right header?

Author: alexcrichton

cmake/modules/HandleLLVMOptions.cmake

@@ -105,9 +105,12 @@ endif()
 
 # Pass -Wl,-z,defs. This makes sure all symbols are defined. Otherwise a DSO
 # build might work on ELF but fail on MachO/COFF.
-if(NOT (${CMAKE_SYSTEM_NAME} MATCHES "Darwin" OR WIN32 OR CYGWIN OR
-        ${CMAKE_SYSTEM_NAME} MATCHES "FreeBSD" OR
-        ${CMAKE_SYSTEM_NAME} MATCHES "OpenBSD") AND
+if(NOT (${TARGET_TRIPLE} MATCHES "darwin" OR
+        ${TARGET_TRIPLE} MATCHES "windows" OR
+        ${TARGET_TRIPLE} MATCHES "mingw" OR
+        ${TARGET_TRIPLE} MATCHES "freebsd" OR
+        ${TARGET_TRIPLE} MATCHES "netbsd" OR
+        ${TARGET_TRIPLE} MATCHES "openbsd") AND
    NOT LLVM_USE_SANITIZER)
   set(CMAKE_SHARED_LINKER_FLAGS "${CMAKE_SHARED_LINKER_FLAGS} -Wl,-z,defs")
 endif()

tools/dsymutil/DwarfLinker.cpp

@@ -1647,7 +1647,7 @@ PointerIntPair<DeclContext *, 1> DeclContextTree::getChildDeclContext(
                         File);
               (void)gotFileName;
               assert(gotFileName && "Must get file name from line table");
-#ifdef HAVE_REALPATH
+#if defined(HAVE_REALPATH) && defined(PATH_MAX)
               char RealPath[PATH_MAX + 1];
               RealPath[PATH_MAX] = 0;
               if (::realpath(File.c_str(), RealPath))