Mach-o MH_EXECUTE rewrite; work-in-progress

On amd64, otool and lldb accept the results.
The entry point points to the unfilter and uncompress routines.
Somehow _start has been lost.

	modified:   p_mach.cpp
	modified:   p_mach.h
	modified:   p_mach_enum.h
	modified:   stub/amd64-darwin.macho-entry.h
	modified:   stub/src/amd64-darwin.macho-entry.S
	modified:   stub/src/amd64-darwin.macho-upxmain.c
	modified:   stub/src/i386-darwin.macho-upxmain.c
	modified:   stub/src/powerpc-darwin.macho-upxmain.c
	modified:   stub/tmp/amd64-darwin.macho-entry.bin.dump

src/p_mach.h

@@ -336,6 +336,7 @@ __packed_struct(Mach_source_version_command)
     Word cmd;  // LC_SOURCE_VERSION
     Word cmdsize;  // 16
     Word version;
+    Word __pad;  // to 0 mod 8
 __packed_struct_end()
 
 template <class TMachITypes>