bazel: CPP fragments in Skylark rule does not provide all fields

Description of the problem / feature request / question:

When using Skylark rules with ‘cpp’ fragments, not all fields that are described in the documentation are accessible.

Here is a simple example: rules.bzl

def _my_rule_impl(ctx):                                    
    print(ctx.fragments.cpp.ar_executable)                 
    print(ctx.fragments.cpp.built_in_include_directories)  
    print(ctx.fragments.cpp.c_options)                     
    print(ctx.fragments.cpp.compiler)                      
    print(ctx.fragments.cpp.compiler_executable)           
    # print(ctx.fragments.cpp.compiler_options)            
    print(ctx.fragments.cpp.cpu)                           
    # print(ctx.fragments.cpp.cxx_options)                 
    # print(ctx.fragments.cpp.dynamic_link_options)        
    # print(ctx.fragments.cpp.fully_static_link_options)   
    print(ctx.fragments.cpp.libc)                          
    print(ctx.fragments.cpp.link_options)                  
    # print(ctx.fragments.cpp.mostly_static_link_options)  
    print(ctx.fragments.cpp.nm_executable)                 
    print(ctx.fragments.cpp.objcopy_executable)            
    print(ctx.fragments.cpp.objdump_executable)            
    print(ctx.fragments.cpp.preprocessor_executable)       
    print(ctx.fragments.cpp.strip_executable)              
    # print(ctx.fragments.cpp.sysroot)                     
    print(ctx.fragments.cpp.target_gnu_system_name)        
    # print(ctx.fragments.cpp.unfiltered_compiler_options) 

my_rule = rule(                                            
    implementation = _my_rule_impl,                        
    fragments = [ 'cpp' ],                                 
)

BUILD

load(':rule.bzl', 'my_rule')

my_rule(
    name = 'testing',
)

All the commented out lines in rules.bzl do not function.

Environment info

• Operating System: Linux

• Bazel version (output of bazel info release): I tried with:

1. release 0.5.3
2. development version (24624776cc23dfacac20eb554ccefe268f3b1a0a)