OS-7973 fbt entry probes incorrectly placed due to -fshrink-wrap

Reviewed by: Jerry Jelinek <jerry.jelinek@joyent.com>
Reviewed by: Robert Mustacchi <rm@joyent.com>
Approved by: Robert Mustacchi <rm@joyent.com>

usr/src/uts/Makefile.uts

@@ -24,7 +24,6 @@
 # Copyright (c) 2011 Bayard G. Bell. All rights reserved.
 # Copyright (c) 2011,2017 by Delphix. All rights reserved.
 # Copyright (c) 2013 Andrew Stormont.  All rights reserved.
-# Copyright 2018 Joyent, Inc.
 # Copyright 2016 Hans Rosenfeld <rosenfeld@grumpf.hope-2000.org>
 # Copyright 2019 Joyent, Inc.
 #
@@ -251,6 +250,12 @@ CFLAGS_uts_i386 += -_gcc7=-mindirect-branch-register
 CFLAGS_uts_i386 += -_gcc8=-mindirect-branch=thunk-extern
 CFLAGS_uts_i386 += -_gcc8=-mindirect-branch-register
 
+#
+# Ensure that the standard function prologue remains at the very start of a function,
+# so DTrace fbt will instrument the right place.
+#
+CFLAGS_uts_i386 = -_gcc=-fno-shrink-wrap
+
 CSTD = $(CSTD_GNU99)
 
 CFLAGS_uts		=