#[link_args] should support runs of spaces

[o11c]

The following uses should work, but currently fail because the linker can't load a file whose name is the empty string.

#[link_args = "-lc "] extern {}

#[link_args = " -lc"] extern {}

#[link_args = "-lm  -lc"] extern {}

Splitting on spaces was introduced in #6192