include/linux/compiler*.h: make compiler-*.h mutually exclusive

Commit cafa0010cd ("Raise the minimum required gcc version to 4.6")
recently exposed a brittle part of the build for supporting non-gcc
compilers.

Both Clang and ICC define __GNUC__, __GNUC_MINOR__, and
__GNUC_PATCHLEVEL__ for quick compatibility with code bases that haven't
added compiler specific checks for __clang__ or __INTEL_COMPILER.

This is brittle, as they happened to get compatibility by posing as a
certain version of GCC.  This broke when upgrading the minimal version
of GCC required to build the kernel, to a version above what ICC and
Clang claim to be.

Rather than always including compiler-gcc.h then undefining or
redefining macros in compiler-intel.h or compiler-clang.h, let's
separate out the compiler specific macro definitions into mutually
exclusive headers, do more proper compiler detection, and keep shared
definitions in compiler_types.h.

Fixes: cafa0010cd ("Raise the minimum required gcc version to 4.6")
Reported-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Suggested-by: Eli Friedman <efriedma@codeaurora.org>
Suggested-by: Joe Perches <joe@perches.com>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/compiler-clang.h

@@ -6,11 +6,7 @@
 /* Some compiler specific definitions are overwritten here
  * for Clang compiler
  */
-
-#ifdef uninitialized_var
-#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
@@ -25,14 +21,8 @@
 #define __SANITIZE_ADDRESS__
 #endif
 
-#undef __no_sanitize_address
 #define __no_sanitize_address __attribute__((no_sanitize("address")))
 
-/* Clang doesn't have a way to turn it off per-function, yet. */
-#ifdef __noretpoline
-#undef __noretpoline
-#endif
-
 /*
  * Not all versions of clang implement the the type-generic versions
  * of the builtin overflow checkers. Fortunately, clang implements
@@ -40,9 +30,17 @@
  * checks. Unfortunately, we don't know which version of gcc clang
  * pretends to be, so the macro may or may not be defined.
  */
-#undef COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW
 #if __has_builtin(__builtin_mul_overflow) && \
     __has_builtin(__builtin_add_overflow) && \
     __has_builtin(__builtin_sub_overflow)
 #define COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW 1
 #endif
+
+/* The following are for compatibility with GCC, from compiler-gcc.h,
+ * and may be redefined here because they should not be shared with other
+ * compilers, like ICC.
+ */
+#define barrier() __asm__ __volatile__("" : : : "memory")
+#define __must_be_array(a) BUILD_BUG_ON_ZERO(__same_type((a), &(a)[0]))
+#define __assume_aligned(a, ...)	\
+	__attribute__((__assume_aligned__(a, ## __VA_ARGS__)))