error[E0463]: can't find crate for `core`- with: `--prefix=/usr --libdir=/usr/lib/amd64`

Rust build fails when using for bootstrap Rust 1.87.0 built with --prefix=/usr --libdir=/usr/lib/amd64 (On Solaris 64 bit libraries need to be placed into /usr/lib/amd64):

Building compiler artifacts (stage0 -> stage1, x86_64-pc-solaris)
warning: `/builds/psumbera/userland-test/components/rust/rustc/build/.cargo/config` is deprecated in favor of `config.toml`
note: if you need to support cargo 1.38 or earlier, you can symlink `config` to `config.toml`
   Compiling proc-macro2 v1.0.93
   Compiling unicode-ident v1.0.14
   Compiling libc v0.2.169
   Compiling cfg-if v1.0.0
error[E0463]: can't find crate for `core`
  |
  = note: the `x86_64-pc-solaris` target may not be installed
  = help: consider downloading the target with `rustup target add x86_64-pc-solaris`
  = help: consider building the standard library from source with `cargo build -Zbuild-std`

For more information about this error, try `rustc --explain E0463`.
error: could not compile `cfg-if` (lib) due to 1 previous error

This used to work with Rust 1.78.0.

Reproduce the bootstrap failure for Rust 1.87.0 on x86_64-pc-solaris with --prefix=/usr and --libdir=/usr/lib/amd64, then compare it with the working Rust 1.78.0 setup. Start from the bootstrap build and target-library lookup involved in the reported error[E0463]; done means the stage0-to-stage1 build finds core and completes successfully with Solaris's 64-bit library layout.