Allow __PAGEZERO.vmsize to be 0x1000 on x86_64, as for "go"-language.

https://github.com/upx/upx/issues/59
	modified:   p_mach.cpp
	modified:   p_mach.h

src/p_mach.h

@@ -855,6 +855,7 @@ protected:
     Mach_segment_command *msegcmd;  // LC_SEGMENT first, without sections
     unsigned o_routines_cmd;  // file offset to LC_ROUINTES
     upx_uint64_t prev_init_address;
+    upx_uint64_t pagezero_vmsize;
     Mach_header mhdri;
 
     Mach_header mhdro;