Fix signal trampolines.

* LP32 should use sa_restorer too. gdb expects this, and future (>= 3.15) x86
  kernels will apparently stop supporting the case where SA_RESTORER isn't
  set.

* gdb and libunwind care about the exact instruction sequences, so we need to
  modify the code slightly in a few cases to match what they're looking for.

* gdb also cares about the exact function names (for some architectures),
  so we need to use __restore and __restore_rt rather than __sigreturn and
  __rt_sigreturn.

* It's possible that we don't have a VDSO; dl_iterate_phdr shouldn't assume
  that getauxval(AT_SYSINFO_EHDR) will return a non-null pointer.

This fixes unwinding through a signal handler in gdb for all architectures.
It doesn't fix libunwind for arm and arm64. I'll keep investigating that...

Bug: 17436734
Change-Id: Ic1ea1184db6655c5d96180dc07bcc09628e647cb
diff --git a/libc/arch-x86/x86.mk b/libc/arch-x86/x86.mk
index a141548..f45eb65 100644
--- a/libc/arch-x86/x86.mk
+++ b/libc/arch-x86/x86.mk
@@ -25,6 +25,8 @@
 libc_bionic_src_files_x86 += \
     arch-x86/bionic/__bionic_clone.S \
     arch-x86/bionic/_exit_with_stack_teardown.S \
+    arch-x86/bionic/__restore_rt.S \
+    arch-x86/bionic/__restore.S \
     arch-x86/bionic/_setjmp.S \
     arch-x86/bionic/setjmp.S \
     arch-x86/bionic/__set_tls.c \