Ignore undefined symbols for -mtls-dialect=gnu2

So it does not fail for arm config that defaults to -mtp=soft (which
issues a call to __aeabi_read_tp).
Reviewed-by: H.J. Lu <hjl.tools@gmail.com>

configure

@@ -7020,7 +7020,7 @@ void foo (void)
 }
 EOF
 if { ac_try='${CC-cc} $CFLAGS $CPPFLAGS -fPIC -mtls-dialect=gnu2 -nostdlib -nostartfiles
-		   conftest.c -o conftest 1>&5'
+		   -shared conftest.c -o conftest 1>&5'
   { { eval echo "\"\$as_me\":${as_lineno-$LINENO}: \"$ac_try\""; } >&5
   (eval $ac_try) 2>&5
   ac_status=$?

configure.ac

@@ -1297,7 +1297,7 @@ void foo (void)
 }
 EOF
 if AC_TRY_COMMAND([${CC-cc} $CFLAGS $CPPFLAGS -fPIC -mtls-dialect=gnu2 -nostdlib -nostartfiles
-		   conftest.c -o conftest 1>&AS_MESSAGE_LOG_FD])
+		   -shared conftest.c -o conftest 1>&AS_MESSAGE_LOG_FD])
 then
   libc_cv_mtls_dialect_gnu2=yes
 else