common.mak: Set default C/C++ standard to gnu11/gnu++11

s390-tools doesn't define a common C/C++ standard at the moment.
Set the standard to gnu11/gnu++11, which is also used by the Kernel, and
establish a common baseline for all tools.

The -std flag is added to ALL_CFLAGS and ALL_CXXFLAGS to avoid losing it
in case CFLAGS are set by an outside entity.
It is also added to CLAGS_FOR_BUILD for this one special cross build
case.

The -std flag is removed from all tools that set it manually until now.

Reviewed-by: Steffen Eiden <seiden@linux.ibm.com>
Reviewed-by: Marc Hartmayer <mhartmay@linux.ibm.com>
Reviewed-by: Benjamin Block <bblock@linux.ibm.com>
Signed-off-by: Jan Höppner <hoeppner@linux.ibm.com>

libpv/Makefile

@@ -21,8 +21,7 @@ WARNINGS := -Wall -Wextra -Wshadow \
 	    -Wno-unused-function -Wno-unused-parameter -Wno-unused-variable \
 	    $(NULL)
 
-ALL_CFLAGS += -std=gnu11 \
-	      -DOPENSSL_API_COMPAT=0x10101000L \
+ALL_CFLAGS += -DOPENSSL_API_COMPAT=0x10101000L \
 	      $(GLIB2_CFLAGS) \
 	      $(LIBCRYPTO_CFLAGS) \
 	      $(LIBCURL_CFLAGS) \