Improve readability and correctness of the OS specific libunwind bits.

All of the access to __exidx_*, dl_iterate_phdr(), etc. is specific to
the findUnwindSections() function. Right now all of the includes and
declarations related to them are scattered throughout the source file.
For example, for <link.h>, we have a full list of operating systems
guarding the #include, even though the code that uses dl_iterate_phdr()
miraculously doesn't use the same list.

Change the code so that findUnwindSections() is preceded by a block of
#ifdefs that share the same structure as the function itself. First
comes all of the macOS specific bits, followed by bare-metal ARM,
followed by ELF EHABI + DWARF.

This actually allows us to build a copy of libunwind without any
specific ifdefs for NetBSD, CloudABI, etc. It likely also unbreaks the
build of libunwind on FreeBSD/armv6, though I can't confirm.

Reviewed by:	compnerd
Differential Revision:	https://reviews.llvm.org/D30696


git-svn-id: https://llvm.org/svn/llvm-project/libunwind/trunk@297174 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/src/AddressSpace.hpp b/src/AddressSpace.hpp
index b15eb5c..23eb933 100644
--- a/src/AddressSpace.hpp
+++ b/src/AddressSpace.hpp
@@ -34,32 +34,6 @@
 #include "dwarf2.h"
 #include "Registers.hpp"
 
-#if _LIBUNWIND_ARM_EHABI
-#if defined(_LIBUNWIND_IS_BAREMETAL)
-// When statically linked on bare-metal, the symbols for the EH table are looked
-// up without going through the dynamic loader.
-extern char __exidx_start;
-extern char __exidx_end;
-#else
-#include <link.h>
-#endif // !defined(_LIBUNWIND_IS_BAREMETAL)
-#endif // _LIBUNWIND_ARM_EHABI
-
-#if defined(__CloudABI__) || defined(__FreeBSD__) || defined(__Fuchsia__) ||  \
-    defined(__linux__) || defined(__NetBSD__)
-#if _LIBUNWIND_SUPPORT_DWARF_UNWIND && _LIBUNWIND_SUPPORT_DWARF_INDEX
-#include <link.h>
-// Macro for machine-independent access to the ELF program headers. This
-// macro is not available on some systems (e.g., FreeBSD). On these
-// systems the data structures are just called Elf_XXX. Define ElfW()
-// locally.
-#if !defined(ElfW)
-#define ElfW(type) Elf_##type
-#endif
-#include "EHHeaderParser.hpp"
-#endif
-#endif
-
 namespace libunwind {
 
 /// Used by findUnwindSections() to return info about needed sections.
@@ -291,6 +265,7 @@
 }
 
 #ifdef __APPLE__ 
+
   struct dyld_unwind_sections
   {
     const struct mach_header*   mh;
@@ -336,6 +311,30 @@
       return true;
     }
   #endif
+
+#elif _LIBUNWIND_ARM_EHABI && defined(_LIBUNWIND_IS_BAREMETAL)
+
+// When statically linked on bare-metal, the symbols for the EH table are looked
+// up without going through the dynamic loader.
+extern char __exidx_start;
+extern char __exidx_end;
+
+#elif _LIBUNWIND_ARM_EHABI || _LIBUNWIND_SUPPORT_DWARF_UNWIND
+
+// ELF-based systems may use dl_iterate_phdr() to access sections
+// containing unwinding information. The ElfW() macro for pointer-size
+// independent ELF header traversal is not provided by <link.h> on some
+// systems (e.g., FreeBSD). On these systems the data structures are
+// just called Elf_XXX. Define ElfW() locally.
+#include <link.h>
+#if !defined(ElfW)
+#define ElfW(type) Elf_##type
+#endif
+
+#if _LIBUNWIND_SUPPORT_DWARF_UNWIND
+#include "EHHeaderParser.hpp"
+#endif
+
 #endif
 
 inline bool LocalAddressSpace::findUnwindSections(pint_t targetAddr,