Invoke "gcc" to build assembler files

[jld01]

Enables pre-processing of *.S and *.sx source files.

Part of #666

[jld01]

I am hopeful that this patch will address concerns about the migration of existing projects when invoking gcc rather than as to build assembly files in managed build projects. My personal opinion is that a switch to using gcc is justified here because the current behaviour is not aligned with user expectations for building *.S and *.sx files.

I have added a command generator for the Assembler flags MBS option that looks for cases where the tool command is gcc but the assembler flags option does not include the -c flag. In such cases:

• The -c flag is added
• Other assembler flags that are not automatically passed to the assembler by the gcc wrapper are prefixed with -Wa,

For example, a project that was previously building assember files using as -K -g would now use gcc -c -Wa,-K -g.

[ruspl-afed]

Looks good for the simple case. And this covers part of the issues raised during the CDT discussion.

And what about [so nice] MBS inheritance? There are three value changes that can participate to the inheritance chain:

• Tool "command"
• Tool "commandGenerator"
• Option "defaultValue"

How do we ensure that this fix will not break downstream integrations?

[jld01]

Thank you the feedback, @ruspl-afed.

And what about [so nice] MBS inheritance? There are three value changes that can participate to the inheritance chain:

* Tool "command"
* Tool "commandGenerator"
* Option "defaultValue"

How do we ensure that this fix will not break downstream integrations?

The latest version of this PR accommodates cases where the tool command has been overridden XOR the option default value has been overridden. There is no issue when both have been overridden. There is no issue when neither has been overridden.

This work does not accommodate the possibility that the tool command generator has been overridden while the tool command and option default value have not been overridden. The risk here seems very low.

[jld01]

@jonahgraham I've invited you to review since there has been some difference in opinion on this topic. I would suggest a N&N entry that covers "how to fix" in extreme edge cases such as overridden Assembler flags option command generator.

[Torbjorn-Svensson]

Sorry to be late the party here, but should this line not match arm-non-eabi-gcc for example, or gcc.exe? Or is it enough to have a strict check for gcc?

[jld01]

We are checking only whether the tool command has been overridden. We don't care if it has been overridden by the user or by an extender. If the tool command has been overridden, then our new default value (gcc) will have no effect on the build and we do not need to modify the assembler flags.