build: compile C code for "xous" operating system

The "xous" operating system is enturely Rust-based, meaning it has no
libc. Therefore, it relies on `compiler-builtins` for all intrinsics.

Unfortunately, there are not yet Rust equivalents for all C functions.
For example, triganometric functions are still missing. In the meantime,
enable C replacements for these functions so that Rust programs compiled
for Xous can call these functions.

Signed-off-by: Sean Cross <sean@xobs.io>

Author: xobs

build.rs

@@ -58,9 +58,11 @@ fn main() {
         //   unlikely that the C is really that much better than our own Rust.
         // * nvptx - everything is bitcode, not compatible with mixed C/Rust
         // * riscv - the rust-lang/rust distribution container doesn't have a C
-        //   compiler nor is cc-rs ready for compilation to riscv (at this
-        //   time). This can probably be removed in the future
-        if !target.contains("wasm") && !target.contains("nvptx") && !target.starts_with("riscv") {
+        //   compiler.
+        if !target.contains("wasm")
+            && !target.contains("nvptx")
+            && (!target.starts_with("riscv") || target.contains("xous"))
+        {
             #[cfg(feature = "c")]
             c::compile(&llvm_target, &target);
         }