Fix `.gitignore`

- Remove all-encompassing `benchmark`
- Add exception for `core:simd/x86`

.gitignore

@@ -17,6 +17,7 @@
 [Rr]eleases/
 x64/
 x86/
+!/core/simd/x86
 bld/
 [Bb]in/
 [Oo]bj/
@@ -31,7 +32,6 @@ tests/documentation/all.odin-doc
 # Uncomment if you have tasks that create the project's static files in wwwroot
 #wwwroot/
 demo
-benchmark
 
 # MSTest test Results
 [Tt]est[Rr]esult*/