Fix unwinding through x86-64 __bionic_clone.

x86-64 needs these CFI directives to stop unwinding here.
I've also cleaned up the assembler a little, and made x86 and x86-64
a little more alike.

Bug: 15195760
Change-Id: I40f92c007843c29c933bb6876fe2b4611e1b946b
diff --git a/libc/arch-x86/bionic/__bionic_clone.S b/libc/arch-x86/bionic/__bionic_clone.S
index 7c972de..672512c 100644
--- a/libc/arch-x86/bionic/__bionic_clone.S
+++ b/libc/arch-x86/bionic/__bionic_clone.S
@@ -25,8 +25,8 @@
         int     $0x80
 
         # Check result.
-        cmpl    $0, %eax
-        je      .L_bc_child
+        testl    %eax, %eax
+        jz      .L_bc_child
         jg      .L_bc_parent
 
         # An error occurred, so set errno and return -1.
@@ -44,7 +44,7 @@
         hlt
 
 .L_bc_parent:
-        # we're the parent; nothing to do.
+        # We're the parent; nothing to do.
 .L_bc_return:
         popl    %edi
         popl    %esi