Implemented user debugging support for x86_64.

Reused x86 arch_user_debugger.cpp, with a few minor changes to make
the code work for both 32 and 64 bit. Something isn't quite working
right, if a breakpoint is hit the kernel will hang. Other than that
everything appears to work correctly.

headers/private/system/syscalls.h

@@ -8,6 +8,7 @@
 
 #include <arch_config.h>
 #include <DiskDeviceDefs.h>
+#include <elf_common.h>
 #include <image.h>
 #include <OS.h>
 
@@ -21,7 +22,6 @@ extern "C" {
 
 struct attr_info;
 struct dirent;
-struct Elf32_Sym;
 struct fd_info;
 struct fd_set;
 struct fs_info;
@@ -226,7 +226,7 @@ extern status_t		_kern_get_image_info(image_id id, image_info *info,
 extern status_t		_kern_get_next_image_info(team_id team, int32 *cookie,
 						image_info *info, size_t size);
 extern status_t		_kern_read_kernel_image_symbols(image_id id,
-						struct Elf32_Sym* symbolTable, int32* _symbolCount,
+						elf_sym* symbolTable, int32* _symbolCount,
 						char* stringTable, size_t* _stringTableSize,
 						addr_t* _imageDelta);