Prevent false passes in gdb.base/vla-optimized-out.exp

The "vla_optimized_out" procedure in gdb.base/vla-optimized-out.exp
accepts a "sizeof_result" argument which is substituted into the
regular expression used to check the result of printing the sizeof
a VLA.  The -O3 test variants, however, pass a regular expression
fragment as that argument, which expands into a regular expression
that matches any result with a "6" in it.  This commit wraps the
substitution with parentheses to prevent these false matches.

gdb/testsuite/ChangeLog:

	* gdb.base/vla-optimized-out.exp (p sizeof (a)): Wrap supplied
	regexp fragment in parentheses to prevent false matching.

gdb/testsuite/ChangeLog

@@ -1,3 +1,8 @@
+2020-11-10  Gary Benson <gbenson@redhat.com>
+
+	* gdb.base/vla-optimized-out.exp (p sizeof (a)): Wrap supplied
+	regexp fragment in parentheses to prevent false matching.
+
 2020-11-10  Gary Benson <gbenson@redhat.com>
 
 	* gdb.base/vla-optimized-out.c (f1): Add __attribute__ ((weak)).

gdb/testsuite/gdb.base/vla-optimized-out.exp

@@ -42,7 +42,7 @@ proc vla_optimized_out {exe_suffix options} {
 	"printed optimized out vla"
 
     gdb_test "p sizeof (a)" \
-	" = $sizeof_result" \
+	" = ($sizeof_result)" \
 	"printed size of optimized out vla"
 
     # At lower optimisation levels, the upper bound of the array is