convert to real Rust triples properly

This should fix conversion to real Rust triples for musl targets, for
i686 targets and possibly others.

classes/rust-triples.bbclass

@@ -12,18 +12,24 @@ def rust_base_triple(d, thing):
     # All the Yocto targets are Linux and are 'unknown'
     vendor = "-unknown"
     os = d.getVar('{}_OS'.format(thing), True)
+    libc = d.getVar('TCLIBC', True)
 
-    if arch.startswith("arm"):
-        if os.endswith("gnueabi"):
-            os += bb.utils.contains('TUNE_FEATURES', 'callconvention-hard', 'hf', '', d)
-    elif arch.startswith("aarch64"):
-        os = "linux-gnu"
-    elif arch.startswith("x86_64"):
-        os = "linux-gnu"
-    elif arch.startswith("i586"):
-        arch = "i686"
-        os = "linux-gnu"
-    return arch + vendor + '-' + os
+    # Prefix with a dash and convert glibc -> gnu
+    if libc == "glibc":
+        libc = "-gnu"
+    elif libc == "musl":
+        libc = "-musl"
+
+    # Don't double up musl (only appears to be the case on aarch64)
+    if os == "linux-musl":
+        if libc != "-musl":
+            bb.fatal("{}_OS was '{}' but TCLIBC was not 'musl'".format(thing, os))
+        os = "linux"
+
+    # This catches ARM targets and appends the necessary hard float bits
+    if os == "linux-gnueabi" or os == "linux-musleabi":
+        libc = bb.utils.contains('TUNE_FEATURES', 'callconvention-hard', 'hf', '', d)
+    return arch + vendor + '-' + os + libc
 
 RUST_BUILD_SYS = "${@rust_base_triple(d, 'BUILD')}"
 RUST_HOST_SYS = "${@rust_base_triple(d, 'HOST')}"