RFR: 8227397: Add --with-extra-asflags configure option

[Florian Weimer]

* Severin Gehwolf:

> Hi Martin,
>
> On Mon, 2019-07-08 at 10:42 -0700, Martin Buchholz wrote:
>> (not really a review ...)
>> 
>> I'm confused because the assembler is invoked when compiling any sort of source file - C, C++, or .s/.S.
>> Wouldn't we want assembler flags passed uniformly, no matter when the assembler is invoked?
>
> Good point. I guess that's debatable. The intention is to only affect
> assembling of, well, assembly files (.s/.S). Right now, --with-extra-
> cflags would work, but that's more an accident I'd think:

Using -Wa,--generate-missing-build-notes=yes for compiling C/C++ files
(as opposed to assembling hand-written assembler input) is problematic
because it more or less invalidates why we have these notes.  They are
supposed to come from the compiler plugin for C/C++ files.  If the
assembler generates them, we won't see in the resulting binary that we
actually have a coverage gap for C/C++ sources because the compiler
plugin wasn't active for some reason.

Thanks,
Florian