Exclude PCH for files in $(OPT_SPEED_SRC) with clang after JDK-8246751

* Reproduce on MacOS with clang
```
bash configure --with-jvm-features=opt-size ...
make images
```

* Symptom
```
Building target 'images' in configuration 'macosx-x86_64-server-release'
Compiling 8 files for BUILD_TOOLS_LANGTOOLS
Compiling 10 properties into resource bundles for jdk.javadoc
Parsing 2 properties into enum-like class for jdk.compiler
Compiling 12 properties into resource bundles for jdk.jdeps
Compiling 7 properties into resource bundles for jdk.jshell
Compiling 1 files for BUILD_JFR_TOOLS
Creating hotspot/variant-server/tools/adlc/adlc from 13 file(s)
Compiling 2 files for BUILD_JVMTI_TOOLS
Compiling 19 properties into resource bundles for jdk.compiler
Compiling 127 files for BUILD_java.compiler.interim
Compiling 405 files for BUILD_jdk.compiler.interim
Compiling 217 files for BUILD_jdk.javadoc.interim
Creating support/modules_libs/java.base/server/libjvm.dylib from 1009 file(s)
error: __OPTIMIZE_SIZE__ predefined macro was enabled in PCH file but is currently disabled
1 error generated.
make[3]: *** [/Users/fool/workspace/open/jdk/build/macosx-x86_64-server-release/hotspot/variant-server/libjvm/objs/allocation.o] Error 1
make[3]: *** Waiting for unfinished jobs....
make[2]: *** [hotspot-server-libs] Error 2
make[2]: *** Waiting for unfinished jobs....

ERROR: Build failed for target 'images' in configuration 'macosx-x86_64-server-release' (exit code 2)

=== Output from failing command(s) repeated here ===
* For target hotspot_variant-server_libjvm_objs_allocation.o:
error: __OPTIMIZE_SIZE__ predefined macro was enabled in PCH file but is currently disabled
1 error generated.

* All command lines available in /Users/fool/workspace/open/jdk/build/macosx-x86_64-server-release/make-support/failure-logs.
=== End of repeated output ===
```