[rust] Figure out how to bow to nightly `panic=immediate-abort` changes

[behdad]

Our rust build is broken again with latest nightly:

   Compiling libm v0.2.15
error: panic_immediate_abort is now a real panic strategy! Enable it with the compiler flags `-Zunstable-options -Cpanic=immediate-abort`
  --> /home/runner/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/panicking.rs:36:1
   |
36 | / compile_error!(
37 | |     "panic_immediate_abort is now a real panic strategy! \
38 | |     Enable it with the compiler flags `-Zunstable-options -Cpanic=immediate-abort`"
39 | | );
   | |_^

I haven't been able to get it to build. Trying the compiler flags that it suggests, I then get:

   Compiling libm v0.2.15
error: the crate `core` was compiled with a panic strategy which is incompatible with `immediate-abort`

Adding core to -Zbuild-std doesn't seem to make a difference.

[behdad]

If anyone wants to try, this is my current patch so far:

diff --git a/src/rust/meson.build b/src/rust/meson.build
index b086dd952..11d8509ca 100644
--- a/src/rust/meson.build
+++ b/src/rust/meson.build
@@ -34,9 +34,9 @@ endif
 
 buildtype = get_option('buildtype')
 if buildtype == 'release' or buildtype == 'debugoptimized'
+  rust_flags += ' -Zunstable-options -Cpanic=immediate-abort'
   cargo_args += [
-    '-Z', 'build-std=std,panic_abort',
-    '-Z', 'build-std-features=panic_immediate_abort',
+    '-Z', 'build-std=core,std,alloc,panic_abort',
   ]
   cargo_args += ['--profile', buildtype]
 endif

[behdad]

Rust PR that introduced this change: rust-lang/rust#146317