kbuild: fix ld-option function

The kbuild's ld-option function is broken because
the command
  $(CC) /dev/null -c -o "$$TMPO"
does not create object file!

I have used a relatively old mips gcc 3.4.6 cross-compiler
and a relatively new gcc 4.7.2 to check this fact
but the results are the same.

EXAMPLE:
  $ rm /tmp/1.o
  $ mips-linux-gcc /dev/null -c -o /tmp/1.o
  mips-linux-gcc: /dev/null: linker input file unused because linking not done
  $ ls -la /tmp/1.o
  ls: cannot access /tmp/1.o: No such file or directory

We can easily fix the problem by adding
the '-x c' compiler option.

EXAMPLE:
  $ rm /tmp/1.o
  $ mips-linux-gcc -x c /dev/null -c -o /tmp/1.o
  $ ls -la /tmp/1.o
  -rw-r--r-- 1 antony antony 778 Apr  2 20:40 /tmp/1.o

Also fix wrong ld-option example.
Signed-off-by: Antony Pavlov <antonynpavlov@gmail.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>

Documentation/kbuild/makefiles.txt

@@ -593,7 +593,7 @@ more details, with real examples.
 
 	Example:
 		#Makefile
-		LDFLAGS_vmlinux += $(call really-ld-option, -X)
+		LDFLAGS_vmlinux += $(call ld-option, -X)
 
 
 === 4 Host Program support

scripts/Kbuild.include

@@ -148,7 +148,7 @@ cc-ldoption = $(call try-run,\
 # ld-option
 # Usage: LDFLAGS += $(call ld-option, -X)
 ld-option = $(call try-run,\
-	$(CC) /dev/null -c -o "$$TMPO" ; $(LD) $(1) "$$TMPO" -o "$$TMP",$(1),$(2))
+	$(CC) -x c /dev/null -c -o "$$TMPO" ; $(LD) $(1) "$$TMPO" -o "$$TMP",$(1),$(2))
 
 # ar-option
 # Usage: KBUILD_ARFLAGS := $(call ar-option,D)