bazel: switch rust LTO from fat to thin

Fixes the `--config=lto` / `--config=pgo-instrument` link, which
started failing with the wasmtime 42 upgrade (rust 1.91) with errors
like:

  ld.lld: error: undefined symbol: __rustc::__rust_no_alloc_shim_is_unstable_v2

The empty `allocator_library` staticlib used by rules_rust's
`experimental_use_allocator_libraries_with_mangled_symbols` exists
only so rustc auto-injects the `__rustc::__rust_*` allocator shims
with GLOBAL binding, satisfying the rust stdlib's GLOBAL undefined
refs to those symbols. Under fat LTO, rustc runs LLVM's Internalize
pass over the staticlib before emitting it; nothing inside the empty
crate references the shims, so they get downgraded GLOBAL -> LOCAL
and ELF then refuses to bind the stdlib's GLOBAL refs to LOCAL defs
across object files. Thin LTO doesn't run that whole-crate
internalization, so the symbols stay GLOBAL and the link succeeds.

No measurable cost: with --config=pgo-instrument the resulting
//:redpanda binary is bit-identical (md5) before and after this
change. Every wasmtime crate is a `rust_library` (rlib), and for
rlibs rules_rust emits `-Clinker-plugin-lto` regardless of the
-Clto mode, so the rlib bitcode is the same either way -- the actual
cross-language LTO happens later at the C++ thin-LTO link step.

Author: travisdowns

.bazelrc

@@ -149,7 +149,22 @@ build:san-all  --linkopt=-fsanitize=address,undefined,vptr,function,alignment
 # PGO + release, but should work with any.
 build:lto --copt -flto=thin
 build:lto --linkopt -flto=thin
-build:lto --@rules_rust//rust/settings:lto=fat
+# Use thin LTO for Rust to match the C++ side. With fat LTO, rustc's
+# LLVM Internalize pass downgrades the auto-injected `__rustc::*`
+# allocator shim symbols from GLOBAL to LOCAL binding when emitting the
+# empty staticlib provided by rules_rust's
+# `experimental_use_allocator_libraries_with_mangled_symbols`. Nothing
+# inside the empty crate references those symbols, so LTO assumes
+# they're internal. The rust stdlib rlibs still emit GLOBAL undefined
+# refs to them, and ELF won't bind a GLOBAL ref to a LOCAL def across
+# object files, so the cc_binary link fails with:
+#   ld.lld: error: undefined symbol: __rustc::__rust_no_alloc_shim_is_unstable_v2
+# Thin LTO doesn't run Internalize across the crate the way fat LTO
+# does, so the symbols stay GLOBAL and the link succeeds. We don't
+# lose any wasm performance: every wasmtime crate is an rlib (rules_rust
+# emits `-Clinker-plugin-lto` regardless of -Clto mode) and the actual
+# cross-language LTO happens at the C++ link step, which is thin too.
+build:lto --@rules_rust//rust/settings:lto=thin
 build:lto --@rules_rust//rust/settings:extra_rustc_flag=-Ccodegen-units=1
 # I've seen others claim these flags help with LTO and the final binary, but these also
 # require a specific version of clang and I don't really want to tie rustc version with