Clean up some misinformation around prctl.

prctl shouldn't be in <unistd.h>.

Change-Id: I29609fc91c033e1ad143b75d9b4eb17aefbd63d6
diff --git a/libc/SYSCALLS.TXT b/libc/SYSCALLS.TXT
index 7ae054e..014d554 100644
--- a/libc/SYSCALLS.TXT
+++ b/libc/SYSCALLS.TXT
@@ -80,9 +80,7 @@
 int     setregid:setregid32(gid_t, gid_t)  arm,x86
 int     setregid:setregid(gid_t, gid_t)    arm64,mips,mips64,x86_64
 int     chroot(const char*)  all
-# IMPORTANT: Even though <sys/prctl.h> declares prctl(int, ...), the syscall stub must take 6 arguments
-#            to match the kernel implementation.
-int     prctl(int option, unsigned int arg2, unsigned int arg3, unsigned int arg4, unsigned int arg5) all
+int     prctl(int, unsigned long, unsigned long, unsigned long, unsigned long) all
 long    __arch_prctl:arch_prctl(int, unsigned long) x86_64
 int     capget(cap_user_header_t header, cap_user_data_t data) all
 int     capset(cap_user_header_t header, const cap_user_data_t data) all