Don't wrap system headers with extern "C"

Implicit wrapping of system header files with  extern "C" is controlled by NO_IMPLICIT_EXTERN_C macro.

This macro is defined in gcc/config/linux.h (gcc/config/gnu-user.h in newer versions of gcc), which included on x86 linux systems (see gcc/config.gcc).

This change defines this macro for nacl.

TEST=see the bug,

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