[libclc] Ensure MACRO_ARCH is always set. (#19971)

Because MACRO_ARCH was not getting set for nvptx or amdgcn, the previous
value of MACRO_ARCH was wrongly being re-used.

Author: hvdijk

libclc/CMakeLists.txt

@@ -472,9 +472,11 @@ foreach( t ${LIBCLC_TARGETS_TO_BUILD} )
     elseif( ARCH STREQUAL nvptx OR ARCH STREQUAL nvptx64 )
       set( build_flags )
       set( opt_flags -O3 "--nvvm-reflect-enable=false" )
+      set( MACRO_ARCH ${ARCH} )
     elseif( ARCH STREQUAL amdgcn )
       set( build_flags )
       set( opt_flags -O3 --amdgpu-oclc-reflect-enable=false )
+      set( MACRO_ARCH ${ARCH} )
     else()
       set( build_flags )
       set( opt_flags -O3 )