sim: add some stdlib.h includes

sim: add some stdlib.h includes

When trying to compile GDB with --target=avr, with gcc 9.2.0, I am
getting a bunch of:

    /home/simark/src/binutils-gdb/sim/avr/../common/nrun.c:94:7: error: implicit declaration of function ‘abort’ [-Werror=implicit-function-declaration]
       94 |       abort ();
          |       ^~~~~
    /home/simark/src/binutils-gdb/sim/avr/../common/nrun.c:94:7: error: incompatible implicit declaration of built-in function ‘abort’ [-Werror]
    /home/simark/src/binutils-gdb/sim/avr/../common/nrun.c:94:7: note: include ‘<stdlib.h>’ or provide a declaration of ‘abort’

I did what the compiler told me and added the relevant includes in the
problematic files.

sim/common/ChangeLog:

	* nrun.c: Include stdlib.h.
	* sim-core.c: Likewise.
	* sim-engine.c: Likewise.
	* sim-io.c: Likewise.
	* sim-module.c: Likewise.
	* sim-reason.c: Likewise.