Define __rust_no_alloc_shim_is_unstable to use our allocator shims

rust-lang/rust#73632 (comment):
```
As of #86844 (scheduled for the 1.71 release) if you are directly
linking the rlibs of the standard library rather than letting rustc
handle linking, you will now need to define a static named
`__rust_no_alloc_shim_is_unstable` which is at least 1 byte big. In
addition if you are using `#[global_allocator]`, you must stop defining
`__rust_alloc`, `__rust_dealloc`, `__rust_realloc` and
`__rust_alloc_zeroed` as they are now directly defined by the
`#[global_allocator]` expansion rather than as part of the allocator
shim. If you are using the default allocator in libstd you will need
to keep defining them though.
```

We use the default allocator, so we can keep using our C++ shims as
long as we define this new symbol.

[email protected]

Bug: 1292038
Change-Id: Ib21c48d8b656155acc0afc6941ed85f819bc9bea
Cq-Include-Trybots: luci.chromium.try:android-rust-arm32-rel,android-rust-arm64-dbg,android-rust-arm64-rel,linux-rust-x64-rel,linux-rust-x64-dbg,win-rust-x64-dbg,mac-rust-x64-dbg,win-rust-x64-rel
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4569204
Reviewed-by: Adrian Taylor <[email protected]>
Commit-Queue: danakj <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1149904}
NOKEYCHECK=True
GitOrigin-RevId: 579b3dd0ea41a40da8a61ab87a8b0bc39e158998

Author: danakj

rust/std/remap_alloc.cc

@@ -75,6 +75,11 @@ extern "C" {
 #define REMAP_ALLOC_ATTRIBUTES __attribute__((weak))
 #endif  // COMPONENT_BUILD
 
+// This must exist as the stdlib depends on it to prove that we know the
+// alloc shims below are unstable. In the future we may be required to replace
+// them with a #[global_allocator] crate (see file comment above for more).
+[[maybe_unused]] unsigned char __rust_no_alloc_shim_is_unstable;
+
 REMAP_ALLOC_ATTRIBUTES void* __rust_alloc(size_t size, size_t align) {
   // This mirrors kMaxSupportedAlignment from
   // base/allocator/partition_allocator/partition_alloc_constants.h.