auto merge of #12084 : alexcrichton/rust/codegen-opts, r=cmr

Move them all behind a new -C switch. This migrates some -Z flags and some
top-level flags behind this -C codegen option.

The -C flag takes values of the form "-C name=value" where the "=value" is
optional for some flags.

Flags affected:

* --llvm-args           => -C llvm-args
* --passes              => -C passes
* --ar                  => -C ar
* --linker              => -C linker
* --link-args           => -C link-args
* --target-cpu          => -C target-cpu
* --target-feature      => -C target-fature
* --android-cross-path  => -C android-cross-path
* --save-temps          => -C save-temps
* --no-rpath            => -C no-rpath
* -Z no-prepopulate     => -C no-prepopulate-passes
* -Z no-vectorize-loops => -C no-vectorize-loops
* -Z no-vectorize-slp   => -C no-vectorize-slp
* -Z soft-float         => -C soft-float
* -Z gen-crate-map      => -C gen-crate-map
* -Z prefer-dynamic     => -C prefer-dynamic
* -Z no-integrated-as   => -C no-integrated-as

As a bonus, this also promotes the -Z extra-debug-info flag to a first class -g
or --debuginfo flag.

* -Z debug-info         => removed
* -Z extra-debug-info   => -g or --debuginfo

Closes #9770
Closes #12000

src/doc/rust.md

@@ -3761,7 +3761,7 @@ dependencies will be used:
    with the above limitations in dynamic and static libraries, it is required
    for all upstream dependencies to be in the same format. The next question is
    whether to prefer a dynamic or a static format. The compiler currently favors
-   static linking over dynamic linking, but this can be inverted with the `-Z
+   static linking over dynamic linking, but this can be inverted with the `-C
    prefer-dynamic` flag to the compiler.
 
    What this means is that first the compiler will attempt to find all upstream