Make gen-syscalls.pl compatible with perl < 5.13.2

An error occurs when trying to build crtools on ARM systems
with perl versions earlier than 5.13.2 due to lack of /r modifier.

% make
...
  GEN      arch/arm/syscalls.S
Bareword found where operator expected at arch/arm/gen-syscalls.pl line 8, near "s/.*include\///gr"
Bareword found where operator expected at arch/arm/gen-syscalls.pl line 10, near "s/.*include\///gr"
Bareword found where operator expected at arch/arm/gen-syscalls.pl line 13, near "s/.*include\///gr"
Bareword found where operator expected at arch/arm/gen-syscalls.pl line 16, near "tr/.-/_/r"
Bareword found where operator expected at arch/arm/gen-syscalls.pl line 17, near "tr/.-/_/r"
syntax error at arch/arm/gen-syscalls.pl line 8, near "s/.*include\///gr"
syntax error at arch/arm/gen-syscalls.pl line 10, near "s/.*include\///gr"
syntax error at arch/arm/gen-syscalls.pl line 13, near "s/.*include\///gr"
syntax error at arch/arm/gen-syscalls.pl line 16, near "tr/.-/_/r"
syntax error at arch/arm/gen-syscalls.pl line 17, near "tr/.-/_/r"
BEGIN not safe after errors--compilation aborted at arch/arm/gen-syscalls.pl line 60.
make[1]: *** No rule to make target `arch/arm/syscalls.S', needed by `arch/arm/syscalls.o'.  Stop.

Signed-off-by: Alexander Kolesen <kolesen.a@gmail.com>
Acked-by: Alexander Kartashov <alekskartashov@parallels.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>