Add arch-x86_64/bionic.

This is basically half of I5de76f6c46ac87779f207d568a86bb453e2414de from
Pavel Chupin <pavel.v.chupin@intel.com>, but with the stock upstream
setjump/sigsetjmp and H.J. Lu's suggested changes to __rt_sigreturn.

Change-Id: I8167ec228faeb2065391e5bec0413cca662f3d33
diff --git a/libc/arch-x86_64/syscalls/arch_prctl.S b/libc/arch-x86_64/syscalls/__arch_prctl.S
similarity index 88%
rename from libc/arch-x86_64/syscalls/arch_prctl.S
rename to libc/arch-x86_64/syscalls/__arch_prctl.S
index eaf9494..21d154c 100644
--- a/libc/arch-x86_64/syscalls/arch_prctl.S
+++ b/libc/arch-x86_64/syscalls/__arch_prctl.S
@@ -3,7 +3,7 @@
 #include <linux/err.h>
 #include <machine/asm.h>
 
-ENTRY(arch_prctl)
+ENTRY(__arch_prctl)
     movl    $__NR_arch_prctl, %eax
     syscall
     cmpq    $-MAX_ERRNO, %rax
@@ -14,4 +14,4 @@
     orq     $-1, %rax
 1:
     ret
-END(arch_prctl)
+END(__arch_prctl)