make: use SYCLCXX as linker for libceed.so when SYCL is enabled

When CC=gcc (as on Aurora), SYCL_FLAG is empty so the libceed.so link
step was using g++ without -fsycl. g++ does not merge SYCL fat binary
device sections from .sycl.cpp objects, leaving libceed.so with only
host-side SYCL stubs. At runtime, the SYCL backend threw
"No kernel named ..." for every kernel, including CeedVectorNorm_Sycl.

Fix: when SYCL_LIB_DIR is detected, switch the libceed.so linker to
$(SYCLCXX) (icpx) and add -fsycl/-fno-sycl-id-queries-fit-in-int to
CEED_LDFLAGS so icpx merges the device images into the shared library.

Also fix PKG_LIBS: use $(filter -fsycl ...,$(SYCLFLAGS)) instead of
$(SYCL_FLAG) so the .pc file carries -fsycl regardless of CC vendor.

Author: pvelesko

Makefile

@@ -597,7 +597,7 @@ ifneq ($(SYCL_DIR),)
   SYCL_LIB_DIR := $(patsubst %/,%,$(dir $(firstword $(SYCL_LIB_DIR))))
 endif
 ifneq ($(SYCL_LIB_DIR),)
-  PKG_LIBS += $(SYCL_FLAG) -lze_loader
+  PKG_LIBS += $(filter -fsycl -fno-sycl-id-queries-fit-in-int,$(SYCLFLAGS)) -lze_loader
   LIBCEED_CONTAINS_CXX = 1
   libceed.sycl  += $(sycl-core.cpp) $(sycl-ref.cpp) $(sycl-shared.cpp) $(sycl-gen.cpp)
   BACKENDS_MAKE += $(SYCL_BACKENDS)
@@ -658,7 +658,12 @@ endif
 
 pkgconfig-libs-private = $(PKG_LIBS)
 ifeq ($(LIBCEED_CONTAINS_CXX),1)
-  $(libceeds) : LINK = $(CXX)
+  ifneq ($(SYCL_LIB_DIR),)
+    $(libceeds) : LINK = $(SYCLCXX)
+    $(libceeds) : CEED_LDFLAGS += $(filter -fsycl -fno-sycl-id-queries-fit-in-int,$(SYCLFLAGS))
+  else
+    $(libceeds) : LINK = $(CXX)
+  endif
   ifeq ($(STATIC),1)
     $(examples) $(tests) : CEED_LDLIBS += $(LIBCXX)
     pkgconfig-libs-private += $(LIBCXX)