llvm: Move some OS selection prongs for clarity.

This makes it clearer which OSs are not known to or supported by LLVM at all vs
the ones where we're intentionally passing choosing "unknown".

src/codegen/llvm.zig

@@ -110,7 +110,7 @@ pub fn targetTriple(allocator: Allocator, target: std.Target) ![]const u8 {
         .netbsd => "netbsd",
         .openbsd => "openbsd",
         .solaris, .illumos => "solaris",
-        .windows => "windows",
+        .windows, .uefi => "windows",
         .zos => "zos",
         .haiku => "haiku",
         .rtems => "rtems",
@@ -128,7 +128,6 @@ pub fn targetTriple(allocator: Allocator, target: std.Target) ![]const u8 {
         .hurd => "hurd",
         .wasi => "wasi",
         .emscripten => "emscripten",
-        .uefi => "windows",
         .macos => "macosx",
         .ios => "ios",
         .tvos => "tvos",
@@ -205,14 +204,7 @@ pub fn targetTriple(allocator: Allocator, target: std.Target) ![]const u8 {
 
 pub fn targetOs(os_tag: std.Target.Os.Tag) llvm.OSType {
     return switch (os_tag) {
-        .freestanding,
-        .other,
-        .opencl,
-        .opengl,
-        .plan9,
-        .contiki,
-        => .UnknownOS,
-
+        .freestanding => .UnknownOS,
         .windows, .uefi => .Win32,
         .dragonfly => .DragonFly,
         .freebsd => .FreeBSD,
@@ -231,6 +223,7 @@ pub fn targetOs(os_tag: std.Target.Os.Tag) llvm.OSType {
         .cuda => .CUDA,
         .nvcl => .NVCL,
         .amdhsa => .AMDHSA,
+        .opencl => .UnknownOS, // https://llvm.org/docs/SPIRVUsage.html#target-triples
         .ps4 => .PS4,
         .ps5 => .PS5,
         .elfiamcu => .ELFIAMCU,
@@ -247,6 +240,12 @@ pub fn targetOs(os_tag: std.Target.Os.Tag) llvm.OSType {
         .shadermodel => .ShaderModel,
         .vulkan => .Vulkan,
         .serenity => .Serenity,
+
+        .opengl,
+        .plan9,
+        .contiki,
+        .other,
+        => .UnknownOS,
     };
 }