servo: Merge #18944 - Stop relying on linking details of std’s default allocator (from servo:jemallocator2); r=nox

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.

Source-Repo: https://github.com/servo/servo
Source-Revision: 07e9794306d597afe5d90d192fd32a99572c3cc3

UltraBlame original commit: 921526150768f9a2e9ba1586a350583f9ad025c9

Author: marco-c

servo/Cargo.lock

@@ -0,0 +1,125 @@
+[
+package
+]
+name
+=
+"
+servo_allocator
+"
+version
+=
+"
+0
+.
+0
+.
+1
+"
+authors
+=
+[
+"
+The
+Servo
+Project
+Developers
+"
+]
+license
+=
+"
+MPL
+-
+2
+.
+0
+"
+publish
+=
+false
+[
+lib
+]
+path
+=
+"
+lib
+.
+rs
+"
+[
+features
+]
+unstable
+=
+[
+"
+kernel32
+-
+sys
+"
+"
+jemallocator
+"
+]
+[
+target
+.
+'
+cfg
+(
+not
+(
+windows
+)
+)
+'
+.
+dependencies
+]
+jemallocator
+=
+{
+version
+=
+"
+0
+.
+1
+.
+3
+"
+optional
+=
+true
+}
+[
+target
+.
+'
+cfg
+(
+windows
+)
+'
+.
+dependencies
+]
+kernel32
+-
+sys
+=
+{
+version
+=
+"
+0
+.
+2
+.
+1
+"
+optional
+=
+true
+}