Introduce sys_sigprocmask and sys_sigaction.

sandbox/linux/system_headers/arm64_linux_ucontext.h

Index: sandbox/linux/system_headers/arm64_linux_ucontext.h
diff --git a/sandbox/linux/system_headers/android_arm64_ucontext.h b/sandbox/linux/system_headers/arm64_linux_ucontext.h
similarity index 78%
rename from sandbox/linux/system_headers/android_arm64_ucontext.h
rename to sandbox/linux/system_headers/arm64_linux_ucontext.h
index df2b66ad2f5174da03e819e57d25a9c0849534a3..46e040759931334285c2e9e6e88ca6f771ecf935 100644
--- a/sandbox/linux/system_headers/android_arm64_ucontext.h
+++ b/sandbox/linux/system_headers/arm64_linux_ucontext.h
@@ -2,8 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-#ifndef SANDBOX_LINUX_SYSTEM_HEADERS_ANDROID_ARM64_UCONTEXT_H_
-#define SANDBOX_LINUX_SYSTEM_HEADERS_ANDROID_ARM64_UCONTEXT_H_
+#ifndef SANDBOX_LINUX_SYSTEM_HEADERS_ARM64_LINUX_UCONTEXT_H_
+#define SANDBOX_LINUX_SYSTEM_HEADERS_ARM64_LINUX_UCONTEXT_H_
 
 #if !defined(__BIONIC_HAVE_UCONTEXT_T)
 #include <asm/sigcontext.h>
@@ -26,4 +26,4 @@ struct ucontext_t {
 #include <sys/ucontext.h>
 #endif  // __BIONIC_HAVE_UCONTEXT_T
 
-#endif  // SANDBOX_LINUX_SYSTEM_HEADERS_ANDROID_ARM64_UCONTEXT_H_
+#endif  // SANDBOX_LINUX_SYSTEM_HEADERS_ARM64_LINUX_UCONTEXT_H_