tools/cmake/common: quote DESCRIPTION in define_overridable_option

The set(..CACHE TYPE DOCSTRING [FORCE]) form treats the docstring as a
single argument, but `${DESCRIPTION}` was being expanded unquoted, so
multi-word descriptions were spilling their trailing words into
subsequent set() arguments. This is latent for the common case because
most existing STRING options are driven by the else-branch (not overridden
via -D), but any STRING option overridden on the cmake command line with
a multi-word description hits it — for example:

  cmake -DEXECUTORCH_VULKAN_FP16_PRECISION=mediump ...

fails with the description text being fed to the downstream shader
generator as CLI args.

One-char fix: quote "${DESCRIPTION}" in both set() calls. Correct for
both single- and multi-word descriptions.

Author: jgibson2

tools/cmake/common/preset.cmake

@@ -82,12 +82,12 @@ macro(define_overridable_option NAME DESCRIPTION VALUE_TYPE DEFAULT_VALUE)
   if(DEFINED ${NAME} AND NOT DEFINED CACHE{${NAME}})
     set(${NAME}
         ${${NAME}}
-        CACHE ${VALUE_TYPE} ${DESCRIPTION} FORCE
+        CACHE ${VALUE_TYPE} "${DESCRIPTION}" FORCE
     )
   else()
     set(${NAME}
         ${DEFAULT_VALUE}
-        CACHE ${VALUE_TYPE} ${DESCRIPTION}
+        CACHE ${VALUE_TYPE} "${DESCRIPTION}"
     )
   endif()