`riscv-rt`: LLVM raises spurious errors in release mode for instructions of ISA extensions (e.g., M or E)

I found this problem first in https://github.com/daniestevez/galileo-osnma/issues/23, but the following self-contained example replicates the problem: https://github.com/daniestevez/riscv/tree/debug-mul-build/riscv-rt/test-build

When this example is built with cargo build --release, I get the following:

   Compiling proc-macro2 v1.0.78
   Compiling unicode-ident v1.0.12
   Compiling syn v1.0.109
   Compiling riscv-rt v0.12.0 (/home/daniel/riscv/riscv-rt)
   Compiling riscv v0.11.0 (/home/daniel/riscv/riscv)
   Compiling embedded-hal v1.0.0
   Compiling critical-section v1.1.2
   Compiling test-build v0.1.0 (/home/daniel/riscv/riscv-rt/test-build)
   Compiling panic-halt v0.2.0
   Compiling quote v1.0.35
   Compiling riscv-rt-macros v0.2.1 (/home/daniel/riscv/riscv-rt/macros)
error: instruction requires the following: 'M' (Integer Multiplication and Division) or 'Zmmul' (Integer Multiplication)
mul t0, t2, t0
^
error: instruction requires the following: 'M' (Integer Multiplication and Division) or 'Zmmul' (Integer Multiplication)
mul t0, t2, t0
^
    Finished release [optimized] target(s) in 2.11s

Despite the error, an executable is produced.

If I build without --release instead, I get:

   Compiling proc-macro2 v1.0.78
   Compiling unicode-ident v1.0.12
   Compiling syn v1.0.109
   Compiling riscv-rt v0.12.0 (/home/daniel/riscv/riscv-rt)
   Compiling riscv v0.11.0 (/home/daniel/riscv/riscv)
   Compiling critical-section v1.1.2
   Compiling embedded-hal v1.0.0
   Compiling test-build v0.1.0 (/home/daniel/riscv/riscv-rt/test-build)
   Compiling panic-halt v0.2.0
   Compiling quote v1.0.35
   Compiling riscv-rt-macros v0.2.1 (/home/daniel/riscv/riscv-rt/macros)
    Finished dev [unoptimized + debuginfo] target(s) in 2.51s

Start with the self-contained example in riscv-rt/test-build and compare cargo build --release with the non-release build. Trace the riscv-rt build path involved in assembling instructions from ISA extensions such as M or E. Done means the release build no longer emits the reported spurious LLVM errors while preserving the expected instruction handling.