Add -Wshorten-64-to-32 flag to mac builds.

src/gdb-jit.cc

Index: src/gdb-jit.cc
diff --git a/src/gdb-jit.cc b/src/gdb-jit.cc
index 2746806e925fe3d1acd65daf97092a4513be9846..2df282224050df3ddf0cdeacc0914da637080f0c 100644
--- a/src/gdb-jit.cc
+++ b/src/gdb-jit.cc
@@ -355,7 +355,7 @@ class ELFSection : public DebugSectionBase<ELFSectionHeader> {
 #if defined(__MACH_O)
 class MachOTextSection : public MachOSection {
  public:
-  MachOTextSection(uintptr_t align,
+  MachOTextSection(uint32_t align,
                    uintptr_t addr,
                    uintptr_t size)
       : MachOSection("__text",
@@ -588,7 +588,8 @@ class MachO BASE_EMBEDDED {
     Writer::Slot<MachOSection::Header> headers =
         w->CreateSlotsHere<MachOSection::Header>(sections_.length());
     cmd->fileoff = w->position();
-    header->sizeofcmds = w->position() - load_command_start;
+    header->sizeofcmds = static_cast<uint32_t>(w->position() -
+        load_command_start);
     for (int section = 0; section < sections_.length(); ++section) {
       sections_[section]->PopulateHeader(headers.at(section));
       sections_[section]->WriteBody(headers.at(section), w);