Check -fvar-tracking via ada_simple_compile

A couple of Ada tests check whether the C compiler supports
-fvar-tracking.  However, this doesn't really work when using
gnat-llvm, because that will invoke clang under the hood.  This patch
arranges to check gnatmake instead, which is more robust even when
toolchains are mix-and-matched.

gdb/testsuite/gdb.ada/array_return.exp

@@ -21,7 +21,7 @@ standard_ada_testfile p
 
 set opts {}
 lappend opts debug
-if { [have_fvar_tracking] } {
+if { [ada_fvar_tracking] } {
     lappend opts "additional_flags=-fvar-tracking"
 }
 

gdb/testsuite/gdb.ada/finish-large.exp

@@ -21,7 +21,7 @@ standard_ada_testfile p
 
 set flags {debug}
 
-if {[have_fvar_tracking]} {
+if {[ada_fvar_tracking]} {
   lappend flags "additional_flags=-fvar-tracking"
 }
 

gdb/testsuite/lib/ada.exp

@@ -263,3 +263,8 @@ proc ada_simple_compile {name options} {
     set output [gdb_compile_ada_1 $src $dest executable $options]
     return [expr {[gdb_compile_test_nofail $output] == 1}]
 }
+
+# Return 1 if GNAT supports -fvar-tracking.
+gdb_caching_proc ada_fvar_tracking {} {
+    return [ada_simple_compile fvar_tracking additional_flags=-fvar-tracking]
+}