target: recognize sh4 architecture in parse_arch()

SH4 (SuperH) is a Rust target tier 3 — sh4-unknown-linux-gnu and
sh4-unknown-redox both exist in upstream rustc as of 1.79+ — but
parse_arch() in cc-rs's target parser doesn't recognise the `sh4`
prefix, so any C-touching crate compiled against an sh4-* target
fails at config time:

    error occurred in cc-rs: target `sh4-unknown-redox` had an
    unknown architecture

Adding the one-line arm here matches the existing convention for
similar small soft-float architectures (mips, loongarch, riscv, bpf).
SH4 has no architecture-specific compile-flag adjustments needed in
cc-rs (no -march or similar), so just normalising the arch name is
sufficient.

Tested by building rustx-sh4 toolchain (rust-src + LLVM-from-source
with SuperH target) end-to-end against the resulting `cc` 1.2.28+sh4;
output is a working `rustnav-broker` SH4 ELF (6.6 MB) running on
sh4-unknown-redox.

Author: dev user

src/target/parser.rs

@@ -168,6 +168,7 @@ fn parse_arch(full_arch: &str) -> Option<&str> {
         arch if arch.starts_with("nvptx") => "nvptx",
 
         arch if arch.starts_with("bpf") => "bpf", // bpfeb | bpfel
+        arch if arch.starts_with("sh4") => "sh4", // sh4 | sh4-unknown-linux-gnu | sh4-unknown-redox
 
         // https://github.com/bytecodealliance/wasmtime/tree/v30.0.1/pulley
         arch if arch.starts_with("pulley64") => "pulley64",