Switch to purely namespaced enums

This breaks code that referred to variant names in the same namespace as
their enum. Reexport the variants in the old location or alter code to
refer to the new locations:

```
pub enum Foo {
    A,
    B
}

fn main() {
    let a = A;
}
```
=>
```
pub use self::Foo::{A, B};

pub enum Foo {
    A,
    B
}

fn main() {
    let a = A;
}
```
or
```
pub enum Foo {
    A,
    B
}

fn main() {
    let a = Foo::A;
}
```

[breaking-change]
auto merge of #18973 : sfackler/rust/enum-namespace-pt2, r=pcwalton

This breaks code that referred to variant names in the same namespace as
their enum. Reexport the variants in the old location or alter code to
refer to the new locations:

```
pub enum Foo {
    A,
    B
}

fn main() {
    let a = A;
}
```
=>
```
pub use self::Foo::{A, B};

pub enum Foo {
    A,
    B
}

fn main() {
    let a = A;
}
```
or
```
pub enum Foo {
    A,
    B
}

fn main() {
    let a = Foo::A;
}
```

[breaking-change]
CFG_CFLAGS is only used for jemalloc, rename all uses to CFG_JEMALLOC_CFLAGS

i686-apple-ios already used CFG_JEMALLOC_CFLAGS, so merge that one
mk/rt/jemalloc: pass CFG_GCCISH_CFLAGS inside CC instead of passing CFG_CFLAGS in EXTRA_CFLAGS

 - CFG_CFLAGS is gone (it was previously only used by jemalloc anyhow).
 - CFG_JEMALLOC_CFLAGS may contain flags needed for the compiler to
   function (produce a binary output).
 - jemalloc's configure runs $(CC) without EXTRA_CFLAGS, and (without
   this change) will fail if any flags are required for CC to work.