package/grpc: work around gcc bug 85180

With Microblaze Gcc version < 8.x the build hangs due to gcc bug
85180: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85180. The bug
shows up when building grpc with optimization but not when building
with -O0. To work around this, if BR2_TOOLCHAIN_HAS_GCC_BUG_85180=y we
force using -O0. Doing this let's optimize already present
CFLAGS/CXXFLAGS tweaking by introducing GRPC_CFLAGS and GRPC_CXXFLAGS
variable.

Fixes:
http://autobuild.buildroot.net/results/6f3/6f301904002cdd50dc3a66fe782b04a05b116319/

Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

package/grpc/grpc.mk

@@ -33,14 +33,25 @@ ifeq ($(BR2_TOOLCHAIN_HAS_LIBATOMIC),y)
 GRPC_CONF_OPTS += -DCMAKE_EXE_LINKER_FLAGS=-latomic
 endif
 
+GRPC_CFLAGS = $(TARGET_CFLAGS)
+GRPC_CXXFLAGS = $(TARGET_CXXFLAGS)
+
 # Set GPR_DISABLE_WRAPPED_MEMCPY otherwise build will fail on x86_64 with uclibc
 # because grpc tries to link with memcpy@GLIBC_2.2.5
 ifeq ($(BR2_x86_64):$(BR2_TOOLCHAIN_USES_GLIBC),y:)
-GRPC_CONF_OPTS += \
-	-DCMAKE_C_FLAGS="$(TARGET_CFLAGS) -DGPR_DISABLE_WRAPPED_MEMCPY" \
-	-DCMAKE_CXX_FLAGS="$(TARGET_CXXFLAGS) -DGPR_DISABLE_WRAPPED_MEMCPY"
+GRPC_CFLAGS += -DGPR_DISABLE_WRAPPED_MEMCPY
+GRPC_CXXFLAGS += -DGPR_DISABLE_WRAPPED_MEMCPY
+endif
+
+ifeq ($(BR2_TOOLCHAIN_HAS_GCC_BUG_85180),y)
+GRPC_CFLAGS += -O0
+GRPC_CXXFLAGS += -O0
 endif
 
+GRPC_CONF_OPTS += \
+	-DCMAKE_C_FLAGS="$(GRPC_CFLAGS)" \
+	-DCMAKE_CXX_FLAGS="$(GRPC_CXXFLAGS)"
+
 HOST_GRPC_CONF_OPTS = \
 	-D_gRPC_CARES_LIBRARIES=cares \
 	-DgRPC_CARES_PROVIDER=none \