Feature request: support setting asm-specific flags

[nicholasbishop]

Background: clang fails with an error if you pass asm options when compiling a non-asm input. For example:

$ clang -c foo.S -Wa,-defsym,abc=0
(no error!)

$ clang -c foo.c -Wa,-defsym,abc=0
error: unknown argument: '-defsym'

It would be helpful to have a Build::asm_flag method for adding flags specific to compiling assembly. The flags passed in this way would only be added to the command for .S/.asm files.

[thomcc]

Patch welcome. I can try to get to it this after thanksgiving if no movement has happened though.

[dot-asm]

Formally speaking, one could have used flag_if_supported("-no-integrated-as") in this case. And one can wonder if it would be generally more appropriate. Indeed, since the suggestion is to pass the option to multiple files(*), it implies that the symbol definition is intended to be global. In which case C is formally entitled to refer to it too :-)

(*) And if not, then it would be more appropriate to split the compilation into two and generate pair of libraries for rustc to link with.