aarch64 fails to build from jdk9/dev.

It fails with an undefined symbol 'os::Linux::ucontext_set_pc'

This was introduced in changset 7995:3eb61269f421

{code}
changeset: 7995:3eb61269f421
user: dholmes
date: Thu Mar 12 19:34:50 2015 -0400
files: src/os/aix/vm/vmError_aix.cpp src/os/bsd/vm/os_bsd.hpp src/os/bsd/vm/vmError_bsd.cpp src/os/linux/vm/os_linux.hpp src/os/linux/vm/vmError_linux.cpp src/os/solaris/vm/os_solaris.hpp src/os/solaris/vm/vmError_solaris.cpp src/os_cpu/aix_ppc/vm/os_aix_ppc.cpp src/os_cpu/bsd_x86/vm/os_bsd_x86.cpp src/os_cpu/bsd_zero/vm/os_bsd_zero.cpp src/os_cpu/linux_ppc/vm/os_linux_ppc.cpp src/os_cpu/linux_sparc/vm/os_linux_sparc.cpp src/os_cpu/linux_x86/vm/os_linux_x86.cpp src/os_cpu/linux_zero/vm/os_linux_zero.cpp src/os_cpu/solaris_sparc/vm/os_solaris_sparc.cpp src/os_cpu/solaris_x86/vm/os_solaris_x86.cpp src/share/vm/runtime/globals.hpp src/share/vm/runtime/stubRoutines.cpp src/share/vm/runtime/stubRoutines.hpp src/share/vm/utilities/vmError.cpp test/runtime/ErrorHandling/SafeFetchInErrorHandlingTest.java
description:
8074552: SafeFetch32 and SafeFetchN do not work in error handling
Reviewed-by: dholmes, goetz
Contributed-by: Thomas Stufe <thomas.stuefe@gmail.com>
{code}

The change was not reflected in the aarch64 specific code.

The attached patch gets it going again.