rustc: fix building emulated x86_64-darwin with jemalloc on aarch64-darwin

Co-authored-by: Randy Eckenrode <randy@largeandhighquality.com>
Co-authored-by: Uri Baghin <uri@canva.com>
Co-authored-by: Winter <winter@winter.cafe>

pkgs/development/compilers/rust/rustc.nix

@@ -136,6 +136,10 @@ in stdenv.mkDerivation rec {
 
     # Useful debugging parameter
     # export VERBOSE=1
+  '' + lib.optionalString (stdenv.isDarwin && stdenv.isx86_64) ''
+    # See https://github.com/jemalloc/jemalloc/issues/1997
+    # Using a value of 48 should work on both emulated and native x86_64-darwin.
+    export JEMALLOC_SYS_WITH_LG_VADDR=48
   '';
 
   # rustc unfortunately needs cmake to compile llvm-rt but doesn't