Allow `[target.'cfg(...)']` syntax in .cargo/config

[mbrubeck]

In Cargo.toml there are two ways to specify target-specific dependencies:

• [target.$triple.dependencies]
• [target.'cfg(...)'.dependencies] (added by Implement cfg-based target-specific dependencies #2328)

But in .cargo/config there is only one way to specify target-specific rustc flags:

• [target.$triple] rustflags = [...] (added by per target rustflags #3153)

It would be useful if [target.'cfg(...)'] was supported here too, for use cases like:

[target.'cfg(target_arch = "x86")']
rustflags = ["-C", "target-feature=+ssse3"]

[alexcrichton]

Seems reasonable to me!