Kbuild: provide a __UNIQUE_ID for clang

The default __UNIQUE_ID macro in compiler.h fails to work for some drivers:

drivers/net/wireless/broadcom/brcm80211/brcmfmac/sdio.c:615:1: error: redefinition of
      '__UNIQUE_ID_firmware615'
BRCMF_FW_NVRAM_DEF(4354, "brcmfmac4354-sdio.bin", "brcmfmac4354-sdio.txt");

This adds a copy of the version we use for gcc-4.3 and higher, as the same
one works with all versions of clang that I could find in svn (2.6 and higher).
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Michal Marek <mmarek@suse.com>

include/linux/compiler-clang.h

@@ -10,3 +10,8 @@
 #undef uninitialized_var
 #define uninitialized_var(x) x = *(&(x))
 #endif
+
+/* same as gcc, this was present in clang-2.6 so we can assume it works
+ * with any version that can compile the kernel
+ */
+#define __UNIQUE_ID(prefix) __PASTE(__PASTE(__UNIQUE_ID_, prefix), __COUNTER__)