re PR go/88060 (../../../gcc-8.2.0/libgo/go/syscall/libcall_linux_utimesnano.go:17:18: error: reference to undefined name ‘_AT_FDCWD’)

PR go/88060
    syscall: always define _AT_FDCWD and IPv6MTUInfo
    
    They aren't defined by old versions of glibc, but are required by the
    code in syscall_linux.go.
    
    Reviewed-on: https://go-review.googlesource.com/c/150697

From-SVN: r266333

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-559fae430b81595efe151222385192a07a9fc3c3
+37cb9763cbe8407b8c3a237b05a5272a226f14a0
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/mksysinfo.sh

@@ -202,6 +202,11 @@ if ! grep '^const AF_LOCAL ' ${OUT} >/dev/null 2>&1; then
   fi
 fi
 
+# The syscall package requires _AT_FDCWD, but doesn't export it.
+if ! grep '^const _AT_FDCWD = ' ${OUT} >/dev/null 2>&1; then
+  echo "const _AT_FDCWD = -100" >> ${OUT}
+fi
+
 # sysconf constants.
 grep '^const __SC' gen-sysinfo.go |
   sed -e 's/^\(const \)__\(SC[^= ]*\)\(.*\)$/\1\2 = __\2/' >> ${OUT}
@@ -669,6 +674,14 @@ grep '^type _ip6_mtuinfo ' gen-sysinfo.go | \
       -e 's/ip6m_mtu/Mtu/' \
     >> ${OUT}
 
+# We need IPv6MTUInfo to compile the syscall package.
+if ! grep 'type IPv6MTUInfo ' ${OUT} >/dev/null 2>&1; then
+  echo 'type IPv6MTUInfo struct { Addr RawSockaddrInet6; Mtu uint32; }' >> ${OUT}
+fi
+if ! grep 'const _sizeof_ip6_mtuinfo = ' ${OUT} >/dev/null 2>&1; then
+  echo 'const SizeofIPv6MTUInfo = 32' >> ${OUT}
+fi
+
 # Try to guess the type to use for fd_set.
 fd_set=`grep '^type _fd_set ' gen-sysinfo.go || true`
 fds_bits_type="_C_long"