build: Move sstream include above safe-ctype.h {PR117771]

build: Move sstream include above safe-ctype.h {PR117771]

sstream in some versions of libstdc++ include locale which might not have been
included yet. safe-ctype.h defines the toupper, tolower, etc. as macros so the
c++ header files needed to be included before hand as comment in system.h says:
/* Include C++ standard headers before "safe-ctype.h" to avoid GCC
   poisoning the ctype macros through safe-ctype.h */

I don't understand how it was working before when memory was included after
safe-ctype.h rather than before. But this makes sstream consistent with the
other C++ headers.

Pushed as obvious after a build for riscv64-elf.

gcc/ChangeLog:

	PR target/117771
	* system.h: Move the include of sstream above safe-ctype.h.

Signed-off-by: Andrew Pinski <quic_apinski@quicinc.com>