gcc-4.9: fixup build with glibc-2.26

Upstream didn't backport this, but fortunately it wasn't too hard.

pkgs/development/compilers/gcc/4.9/default.nix

@@ -72,6 +72,7 @@ let version = "4.9.4";
       # target libraries and tools.
       ++ optional langAda ../gnat-cflags.patch
       ++ optional langFortran ../gfortran-driving.patch
+      ++ [ ../struct-ucontext.patch ../struct-sigaltstack-4.9.patch ] # glibc-2.26
       ;
 
     javaEcj = fetchurl {

pkgs/development/compilers/gcc/struct-sigaltstack-4.9.patch

@@ -0,0 +1,78 @@
+hand-resolved trivial conflicts for 4.9 from the upstream patch
+72edc2c02f8b4768ad660f46a1c7e2400c0a8e06
+diff --git a/libsanitizer/sanitizer_common/sanitizer_linux.cc b/libsanitizer/sanitizer_common/sanitizer_linux.cc
+index 69c9c10..8e53673 100644
+--- a/libsanitizer/sanitizer_common/sanitizer_linux.cc
++++ b/libsanitizer/sanitizer_common/sanitizer_linux.cc
+@@ -599,8 +599,7 @@ uptr internal_prctl(int option, uptr arg2, uptr arg3, uptr arg4, uptr arg5) {
+   return internal_syscall(__NR_prctl, option, arg2, arg3, arg4, arg5);
+ }
+ 
+-uptr internal_sigaltstack(const struct sigaltstack *ss,
+-                         struct sigaltstack *oss) {
++uptr internal_sigaltstack(const void *ss, void *oss) {
+   return internal_syscall(__NR_sigaltstack, (uptr)ss, (uptr)oss);
+ }
+ 
+diff --git a/libsanitizer/sanitizer_common/sanitizer_linux.h b/libsanitizer/sanitizer_common/sanitizer_linux.h
+index 6422df1..8e111d1 100644
+--- a/libsanitizer/sanitizer_common/sanitizer_linux.h
++++ b/libsanitizer/sanitizer_common/sanitizer_linux.h
+@@ -18,7 +18,6 @@
+ #include "sanitizer_platform_limits_posix.h"
+ 
+ struct link_map;  // Opaque type returned by dlopen().
+-struct sigaltstack;
+ 
+ namespace __sanitizer {
+ // Dirent structure for getdents(). Note that this structure is different from
+@@ -28,8 +27,7 @@ struct linux_dirent;
+ // Syscall wrappers.
+ uptr internal_getdents(fd_t fd, struct linux_dirent *dirp, unsigned int count);
+ uptr internal_prctl(int option, uptr arg2, uptr arg3, uptr arg4, uptr arg5);
+-uptr internal_sigaltstack(const struct sigaltstack* ss,
+-                          struct sigaltstack* oss);
++uptr internal_sigaltstack(const void* ss, void* oss);
+ uptr internal_sigaction(int signum, const __sanitizer_kernel_sigaction_t *act,
+     __sanitizer_kernel_sigaction_t *oldact);
+ uptr internal_sigprocmask(int how, __sanitizer_kernel_sigset_t *set,
+diff --git a/libsanitizer/sanitizer_common/sanitizer_stoptheworld_linux_libcdep.cc b/libsanitizer/sanitizer_common/sanitizer_stoptheworld_linux_libcdep.cc
+index 891386dc..234e8c6 100644
+--- a/libsanitizer/sanitizer_common/sanitizer_stoptheworld_linux_libcdep.cc
++++ b/libsanitizer/sanitizer_common/sanitizer_stoptheworld_linux_libcdep.cc
+@@ -273,7 +273,7 @@ static int TracerThread(void* argument) {
+ 
+   // Alternate stack for signal handling.
+   InternalScopedBuffer<char> handler_stack_memory(kHandlerStackSize);
+-  struct sigaltstack handler_stack;
++  stack_t handler_stack;
+   internal_memset(&handler_stack, 0, sizeof(handler_stack));
+   handler_stack.ss_sp = handler_stack_memory.data();
+   handler_stack.ss_size = kHandlerStackSize;
+diff --git a/libsanitizer/tsan/tsan_platform_linux.cc b/libsanitizer/tsan/tsan_platform_linux.cc
+index 2ed5718..6f972ab 100644
+--- a/libsanitizer/tsan/tsan_platform_linux.cc
++++ b/libsanitizer/tsan/tsan_platform_linux.cc
+@@ -287,7 +287,7 @@ void InitializePlatform() {
+ int ExtractResolvFDs(void *state, int *fds, int nfd) {
+ #if SANITIZER_LINUX && !SANITIZER_ANDROID
+   int cnt = 0;
+-  __res_state *statp = (__res_state*)state;
++  struct __res_state *statp = (struct __res_state*)state;
+   for (int i = 0; i < MAXNS && cnt < nfd; i++) {
+     if (statp->_u._ext.nsaddrs[i] && statp->_u._ext.nssocks[i] != -1)
+       fds[cnt++] = statp->_u._ext.nssocks[i];
+
+error: 'SIGSEGV' was not declared in this scope
+diff --git a/libsanitizer/asan/asan_linux.cc b/libsanitizer/asan/asan_linux.cc
+index 0692eb1..472f734 100644
+--- a/libsanitizer/asan/asan_linux.cc
++++ b/libsanitizer/asan/asan_linux.cc
+@@ -26,6 +26,7 @@
+ #include <sys/types.h>
+ #include <fcntl.h>
+ #include <pthread.h>
++#include <signal.h>
+ #include <stdio.h>
+ #include <unistd.h>
+ #include <unwind.h>