remove __arch_prctl alias for arch_prctl

if there's evidence of any use for it, we can add it back later. as
far as I can tell, glibc has it only for internal use (and musl uses a
direct syscall in that case rather than a function call), not for
exposing it to applications.

src/linux/arch_prctl.c

 #include "syscall.h"
 #ifdef SYS_arch_prctl
-#include "libc.h"
-int __arch_prctl(int code, unsigned long addr)
+int arch_prctl(int code, unsigned long addr)
 {
 	return syscall(SYS_arch_prctl, code, addr);
 }
-weak_alias(__arch_prctl, arch_prctl);
 #endif