makefiles/libc: fix wchar_t size warnings on LLVM

The test for the requirement of disabling wchar_t size warnings assumed
that $(CC) is used for linking, instead of $(LINK). With GCC $(LINK)
and $(CC) happen to be (in most cases) identical, but with LLVM they
are not. This results in issues with compiling with LLVM.

makefiles/libc/newlib.mk

@@ -2,7 +2,7 @@ ifneq (,$(filter newlib_nano,$(USEMODULE)))
   # Test if nano.specs is available
   ifeq ($(shell $(LINK) -specs=nano.specs -E - 2>/dev/null >/dev/null </dev/null ; echo $$?),0)
     USE_NEWLIB_NANO = 1
-    ifeq ($(shell echo "int main(){} void _exit(int n) {(void)n;while(1);}" | LC_ALL=C $(CC) -xc - -o /dev/null -lc -specs=nano.specs -Wall -Wextra -pedantic 2>&1 | grep -q "use of wchar_t values across objects may fail" ; echo $$?),0)
+    ifeq ($(shell echo "int main(){} void _exit(int n) {(void)n;while(1);}" | LC_ALL=C $(LINK) -xc - -o /dev/null -lc -specs=nano.specs -Wall -Wextra -pedantic 2>&1 | grep -q "use of wchar_t values across objects may fail" ; echo $$?),0)
         CFLAGS += -fshort-wchar
         LINKFLAGS += -Wl,--no-wchar-size-warning
     endif

makefiles/libc/picolibc.mk

@@ -2,7 +2,7 @@ ifneq (,$(filter picolibc,$(USEMODULE)))
   # Test if picolibc.specs is available
   ifeq ($(shell $(LINK) -specs=picolibc.specs -E - 2>/dev/null >/dev/null </dev/null ; echo $$?),0)
     USE_PICOLIBC = 1
-    ifeq ($(shell echo "int main(){} void _exit(int n) {(void)n;while(1);}" | LC_ALL=C $(CC) -xc - -o /dev/null -lc -specs=picolibc.specs -Wall -Wextra -pedantic 2>&1 | grep -q "use of wchar_t values across objects may fail" ; echo $$?),0)
+    ifeq ($(shell echo "int main(){} void _exit(int n) {(void)n;while(1);}" | LC_ALL=C $(LINK) -xc - -o /dev/null -lc -specs=picolibc.specs -Wall -Wextra -pedantic 2>&1 | grep -q "use of wchar_t values across objects may fail" ; echo $$?),0)
         CFLAGS += -fshort-wchar
         LINKFLAGS += -Wl,--no-wchar-size-warning
     endif