kbuild: add macro for controlling warnings to linux/compiler.h

kbuild: add macro for controlling warnings to linux/compiler.h

I have occasionally run into a situation where it would make sense to
control a compiler warning from a source file rather than doing so from
a Makefile using the $(cc-disable-warning, ...) or $(cc-option, ...)
helpers.

The approach here is similar to what glibc uses, using __diag() and
related macros to encapsulate a _Pragma("GCC diagnostic ...") statement
that gets turned into the respective "#pragma GCC diagnostic ..." by
the preprocessor when the macro gets expanded.

Like glibc, I also have an argument to pass the affected compiler
version, but decided to actually evaluate that one. For now, this
supports GCC_4_6, GCC_4_7, GCC_4_8, GCC_4_9, GCC_5, GCC_6, GCC_7,
GCC_8 and GCC_9. Adding support for CLANG_5 and other interesting
versions is straightforward here. GNU compilers starting with gcc-4.2
could support it in principle, but "#pragma GCC diagnostic push"
was only added in gcc-4.6, so it seems simpler to not deal with those
at all. The same versions show a large number of warnings already,
so it seems easier to just leave it at that and not do a more
fine-grained control for them.

The use cases I found so far include:

- turning off the gcc-8 -Wattribute-alias warning inside of the
  SYSCALL_DEFINEx() macro without having to do it globally.

- Reducing the build time for a simple re-make after a change,
  once we move the warnings from ./Makefile and
  ./scripts/Makefile.extrawarn into linux/compiler.h

- More control over the warnings based on other configurations,
  using preprocessor syntax instead of Makefile syntax. This should make
  it easier for the average developer to understand and change things.

- Adding an easy way to turn the W=1 option on unconditionally
  for a subdirectory or a specific file. This has been requested
  by several developers in the past that want to have their subsystems
  W=1 clean.

- Integrating clang better into the build systems. Clang supports
  more warnings than GCC, and we probably want to classify them
  as default, W=1, W=2 etc, but there are cases in which the
  warnings should be classified differently due to excessive false
  positives from one or the other compiler.

- Adding a way to turn the default warnings into errors (e.g. using
  a new "make E=0" tag) while not also turning the W=1 warnings into
  errors.

This patch for now just adds the minimal infrastructure in order to
do the first of the list above. As the #pragma GCC diagnostic
takes precedence over command line options, the next step would be
to convert a lot of the individual Makefiles that set nonstandard
options to use __diag() instead.

[paul.burton@mips.com:
  - Rebase atop current master.
  - Add __diag_GCC, or more generally __diag_<compiler>, abstraction to
    avoid code outside of linux/compiler-gcc.h needing to duplicate
    knowledge about different GCC versions.
  - Add a comment argument to __diag_{ignore,warn,error} which isn't
    used in the expansion of the macros but serves to push people to
    document the reason for using them - per feedback from Kees Cook.
  - Translate severity to GCC-specific pragmas in linux/compiler-gcc.h
    rather than using GCC-specific in linux/compiler_types.h.
  - Drop all but GCC 8 macros, since we only need to define macros for
    versions that we need to introduce pragmas for, and as of this
    series that's just GCC 8.
  - Capitalize comments in linux/compiler-gcc.h to match the style of
    the rest of the file.
  - Line up macro definitions with tabs in linux/compiler-gcc.h.]

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Tested-by: Christophe Leroy <christophe.leroy@c-s.fr>
Tested-by: Stafford Horne <shorne@gmail.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>