Fix syscall preprocessing

'rizin' is missing a lot of syscalls for i.e. arm. We can check that
with:

 rizin -a x86_64 -b 64 -k linux -q -c asl

vs

 rizin -a arm -b 64 -k linux -q -c asl

'rz_syscall_item_new_from_string' splits the incoming string by commas
and expects at least 3 items as a result. This patch fixes
syscall preprocessing by appending empty strings to match the expected
number of items after a split.

This is a clone of a fix posted to radare2 [1].

[1] https://github.com/radareorg/radare2/pull/21508

sys/syscall_preprocessing.py

@@ -12,6 +12,16 @@ with open(sys.argv[1], encoding="utf8") as inf:
     with open(sys.argv[2], "w", encoding="utf8") as outf:
         for line in inf:
             if not line.startswith("_") and "=" in line:
-                arr = re.split("=|,", line)
+                arr = [el.strip() for el in re.split("=|,", line)]
                 print("%s.%s=%s" % (arr[1], arr[2], arr[0]), file=outf)
-            print(line, file=outf, end="")
+
+                # Can't just append the original line, because
+                # rz_syscall_item_new_from_string splits it by commas
+                # and wants at least 3 items in the result, whereas
+                # original lines, at least in some archs, have only
+                # two items.
+                while len(arr) < 4:
+                    arr.append("")
+                print("%s=%s" % (arr[0], ",".join(arr[1:])), file=outf)
+            else:
+                print(line, file=outf, end="")

test/db/analysis/arm64

@@ -24,6 +24,21 @@ call
 EOF
 RUN
 
+NAME=syscall
+FILE==
+CMDS=<<EOF
+e asm.arch=arm
+e asm.bits=64
+e asm.os=linux
+asn fstat
+asr 80
+EOF
+EXPECT=<<EOF
+80
+fstat
+EOF
+RUN
+
 NAME=noreturn errno
 FILE=bins/mach0/BatteryLife.arm_64.1
 CMDS=<<EOF