fix(arch): narrow file names by go/build's suffix rule

Assisted-by: GLM 5.3 Flash

arch/arch_test.go

@@ -7,13 +7,40 @@ import "testing"
 
 func TestFromFilename(t *testing.T) {
 	cases := map[string]Arch{
+		// Positive: the final underscore segment is the architecture.
 		"avx2_amd64.s":     AMD64,
 		"foo_arm64.s":      ARM64,
-		"portable.s":       Unknown,
-		"decode_ARM64.S":   ARM64,
 		"kernels_amd64.s":  AMD64,
 		"kernel_riscv64.s": RISCV,
 		"kernel_loong64.s": LOONG64,
+		"decode_arm64.S":   ARM64, // the extension is cut with the first dot
+		// OS before arch: the _<os>_<arch> pair form.
+		"sys_darwin_arm64.s": ARM64,
+		"rt0_linux_amd64.s":  AMD64,
+		// A trailing _test segment is dropped before the suffix rule.
+		"foo_amd64_test.s": AMD64,
+		// Trailing junk: a final segment that merely contains an
+		// architecture name narrows nothing, exactly as go/build's
+		// segment rule says.
+		"x_loong64y.s":          Unknown,
+		"asm_amd64x.s":          Unknown, // a real GOROOT name
+		"asm_darwin_arm64_gc.s": Unknown, // the trailing _gc segment is junk
+		"x_loong.s":             Unknown,
+		"foo_amd64x_test.s":     Unknown,
+		// Case sensitivity: the segment match is exact, as go/build's is.
+		"foo_AMD64.s":    Unknown,
+		"decode_ARM64.S": Unknown,
+		// No underscore at all: generic whatever the stem says.
+		"amd64.s":    Unknown,
+		"portable.s": Unknown,
+		// A GOOS-only name narrows by operating system, not architecture.
+		"foo_linux.s": Unknown,
+		// An architecture gasm does not assemble for: named, but Unknown.
+		"vlop_arm.s":  Unknown,
+		"foo_riscv.s": Unknown,
+		// Directories are stripped first, whatever dots they carry.
+		"some/dir/kernel_loong64.s": LOONG64,
+		"/a.b/x_amd64.s":            AMD64,
 	}
 	for name, want := range cases {
 		if got := FromFilename(name); got != want {