Stop relying on linking details of std’s default allocator

[SimonSapin]

We’ve been bitten before by symbol names changing: servo/heapsize#46, and upstream is planning to stop using jemalloc by default: rust-lang/rust#33082 (comment)

So use the (relatively) new #[global_allocator] attribute to explicitly select the system allocator on Windows and jemalloc (now in an external crate) on other platforms. This choice matches current defaults.

---

This change is 

[jdm]

From appveyor:

    Running `rustc --crate-name servo_allocator C:\projects\servo\components\allocator\lib.rs --crate-type lib --emit=dep-info,link -C codegen-units=4 -C debuginfo=2 --cfg "feature=\"jemallocator\"" --cfg "feature=\"kernel32-sys\"" --cfg "feature=\"unstable\"" -C metadata=1d3d51af39724855 -C extra-filename=-1d3d51af39724855 --out-dir C:\projects\servo\target\debug\deps -L dependency=C:\projects\servo\target\debug\deps --extern kernel32=C:\projects\servo\target\debug\deps\libkernel32-b218c0c26de4e27c.rlib -W unused-extern-crates`
error: use of unstable library feature 'allocator_api' (see issue #32838)
  --> C:\projects\servo\components\allocator\lib.rs:11:15
   |
11 | static ALLOC: platform::Allocator = platform::Allocator;
   |               ^^^^^^^^^^^^^^^^^^^
   |
   = help: add #![feature(allocator_api)] to the crate attributes to enable
error: use of unstable library feature 'allocator_api' (see issue #32838)
  --> C:\projects\servo\components\allocator\lib.rs:11:37
   |
11 | static ALLOC: platform::Allocator = platform::Allocator;
   |                                     ^^^^^^^^^^^^^^^^^^^
   |
   = help: add #![feature(allocator_api)] to the crate attributes to enable
error: use of unstable library feature 'alloc_system': this library is unlikely to be stabilized in its current form or name (see issue #27783)
  --> C:\projects\servo\components\allocator\lib.rs:31:5
   |
31 |     extern crate alloc_system;
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = help: add #![feature(alloc_system)] to the crate attributes to enable
error: use of unstable library feature 'allocator_api' (see issue #32838)
  --> C:\projects\servo\components\allocator\lib.rs:34:13
   |
34 |     pub use self::alloc_system::System as Allocator;
   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = help: add #![feature(allocator_api)] to the crate attributes to enable
error: aborting due to 4 previous errors
error: Could not compile `servo_allocator`.

[SimonSapin]

Thanks Josh. Fixed.

[nox]

@bors-servo r+

[bors-servo]

📌 Commit 959ce48 has been approved by nox

[bors-servo]

⌛ Testing commit 959ce48 with merge e58a9f1...

[bors-servo]

💔 Test failed - linux-rel-css

[jdm]

@bors-servo: retry

• #18950 #18949

[bors-servo]

⌛ Testing commit 959ce48 with merge 07e9794...

[bors-servo]

☀️ Test successful - android, arm32, arm64, linux-dev, linux-rel-css, linux-rel-wpt, mac-dev-unit, mac-rel-css1, mac-rel-css2, mac-rel-wpt1, mac-rel-wpt2, mac-rel-wpt3, mac-rel-wpt4, windows-msvc-dev
Approved by: nox
Pushing 07e9794 to master...

[glennw]

@jdm Were #18950 and #18949 opened due to crashes that started happening with this PR? I think they may be genuine crashes caused by this PR...

[jdm]

@glennw Probably? That's good to know!

[SimonSapin]

See #19011 and #19011 (comment).