riscv/bug: Add ARCH_WARN_ASM macro for BUG/WARN asm code sharing with Rust

riscv/bug: Add ARCH_WARN_ASM macro for BUG/WARN asm code sharing with Rust

Add new ARCH_WARN_ASM macro for BUG/WARN assembly code sharing with
Rust to avoid the duplication.

No functional changes.

Acked-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
Link: https://lore.kernel.org/r/20250502094537.231725-3-fujita.tomonori@gmail.com
[ Remove ending newline in `ARCH_WARN_ASM` content to be closer to the
  original. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>