How to define 'srcs' based on OS or toolchain?

How can I add a source file to the srcs list based on the target OS or CPU?

```
cc_library(
  ...
  srcs = [
    "file_util.cc",
    "file_util_posix.cc" if OS==LINUX else "",
    "file_util_win.cc" if OS==WIN else "",
    ...
    "widget.c",
    "widget_x86.c" if CPU==X86 else "",
    "widget_arm.c" if CPU==ARM else "",
  ],
  ...
)
```
