[PATCH] crypto: Fix build for clang

[Mathieu Malaterre]

The header file `longlong.h` makes uses of GNU extensions, this trigger
an error when compiling this code with clang. Add a special flag to make
clang tolerate this syntax.

Silence the following warnings triggered using W=1:

    CC      lib/mpi/generic_mpih-mul1.o
  ../lib/mpi/generic_mpih-mul1.c:37:13: error: invalid use of a cast in a inline asm context requiring an l-value: remove the cast or build with
        -fheinous-gnu-extensions
                  umul_ppmm(prod_high, prod_low, s1_ptr[j], s2_limb);
                  ~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  ../lib/mpi/longlong.h:824:20: note: expanded from macro 'umul_ppmm'
          : "=r" ((USItype) ph) \
                  ~~~~~~~~~~^~

and

    CC      lib/mpi/generic_mpih-mul2.o
  ../lib/mpi/generic_mpih-mul2.c:36:13: error: invalid use of a cast in a inline asm context requiring an l-value: remove the cast or build with
        -fheinous-gnu-extensions
                  umul_ppmm(prod_high, prod_low, s1_ptr[j], s2_limb);
                  ~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  ../lib/mpi/longlong.h:824:20: note: expanded from macro 'umul_ppmm'
          : "=r" ((USItype) ph) \
                  ~~~~~~~~~~^~

  1 warning generated.
    CC      lib/mpi/generic_mpih-mul3.o
  ../lib/mpi/generic_mpih-mul3.c:36:13: error: invalid use of a cast in a inline asm context requiring an l-value: remove the cast or build with
        -fheinous-gnu-extensions
                  umul_ppmm(prod_high, prod_low, s1_ptr[j], s2_limb);
                  ~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  ../lib/mpi/longlong.h:824:20: note: expanded from macro 'umul_ppmm'
          : "=r" ((USItype) ph) \
                  ~~~~~~~~~~^~

Or even:

  ../lib/mpi/mpih-div.c:99:16: error: invalid use of a cast in a inline asm context requiring an l-value: remove the cast or build with -fheinous-gnu-extensions
                                  sub_ddmmss(n1, n0, n1, n0, d1, d0);
                                  ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~

Cc: Joel Stanley <joel@xxxxxxxxx>
Cc: Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx>
Signed-off-by: Mathieu Malaterre <malat@xxxxxxxxxx>
---
 lib/mpi/Makefile | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/lib/mpi/Makefile b/lib/mpi/Makefile
index d5874a7f5ff9..de4d96e988a3 100644
--- a/lib/mpi/Makefile
+++ b/lib/mpi/Makefile
@@ -5,6 +5,13 @@
 
 obj-$(CONFIG_MPILIB) = mpi.o
 
+ifdef CONFIG_CC_IS_CLANG
+CFLAGS_generic_mpih-mul1.o  += -fheinous-gnu-extensions
+CFLAGS_generic_mpih-mul2.o  += -fheinous-gnu-extensions
+CFLAGS_generic_mpih-mul3.o  += -fheinous-gnu-extensions
+CFLAGS_mpih-div.o  += -fheinous-gnu-extensions
+endif
+
 mpi-y = \
 	generic_mpih-lshift.o		\
 	generic_mpih-mul1.o		\
-- 
2.20.1