ARM: only enable C++11 for libroot, not globally

This resembles the x86_64 build, and fixes the compilation issues
with the bootloader stdio.h header.

Fixes #11144

build/jam/ArchitectureRules

@@ -45,9 +45,6 @@ rule ArchitectureSetup architecture
 	if $(cpu) = arm {
 		# For stackcrawls
 		gccBaseFlags += -mapcs-frame ;
-
-		# C++11 is used to implement atomics.
-		HAIKU_C++FLAGS_$(architecture) += -std=c++11 ;
 	}
 
 	# activating graphite optimizations

src/system/libroot/os/arch/arm/Jamfile

@@ -1,5 +1,7 @@
 SubDir HAIKU_TOP src system libroot os arch arm ;
 
+SubDirC++Flags -std=gnu++11 ;
+
 local architectureObject ;
 for architectureObject in [ MultiArchSubDirSetup arm ] {
 	on $(architectureObject) {