Revert "Default to use llvm16"

This reverts commit bea72feae3cf98eafd793719150c3ec57a133b8e.

Reason for revert: Breaks the Swiftshader -> Dawn roll:
```
undefined symbol: public: virtual __cdecl llvm::MCSymbolizer::~MCSymbolizer(void)
```

Change-Id: I1d8855f9b9149825309c604ddfcdee379752b8e4
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/77428
Tested-by: Shahbaz Youssefi <syoussefi@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@google.com>
Reviewed-by: Geoff Lang <geofflang@google.com>
Commit-Queue: Shahbaz Youssefi <syoussefi@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>

src/Reactor/BUILD.gn

diff --git a/src/Reactor/BUILD.gn b/src/Reactor/BUILD.gn
index c2e894a..67dfeb0 100644
--- a/src/Reactor/BUILD.gn
+++ b/src/Reactor/BUILD.gn
@@ -307,11 +307,7 @@
 
 if (supports_llvm) {
   swiftshader_source_set("swiftshader_llvm_reactor") {
-    if (target_os != "fuchsia") {
-        llvm_dir = "../../third_party/llvm-16.0"
-    } else {
-        llvm_dir = "../../third_party/llvm-10.0"
-    }
+    llvm_dir = "../../third_party/llvm-10.0"
 
     deps = [
       ":swiftshader_reactor_base",