[gdb/testsuite] xfail gdb.ada/O2_float_param.exp for aarch64 and gcc 7.5.0

On aarch64-linux, with gcc 7.5.0, we run into:
...
 (gdb) frame^M
 #0  callee.increment (val=99.0, val@entry=9.18340949e-41, msg=...) at \
   callee.adb:21^M
 21            if Val > 200.0 then^M
 (gdb) FAIL: gdb.ada/O2_float_param.exp: scenario=all: frame
...

The problem is a GCC bug, filed as "PR98148 - [AArch64] Wrong location
expression for function entry values" (
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98148 ).

Xfail the test for aarch64 and gcc 7.

Tested on x86_64-linux and aarch64-linux.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29418

gdb/testsuite/gdb.ada/O2_float_param.exp

@@ -19,6 +19,13 @@ if { [skip_ada_tests] } { return -1 }
 
 standard_ada_testfile foo
 
+set have_xfail 0
+if { [is_aarch64_target] } then {
+    if { [gcc_major_version] <= 8 } {
+       set have_xfail 1
+    }
+}
+
 foreach_with_prefix scenario {all minimal} {
     set flags [list debug \
 		   optimize=-O2 \
@@ -32,6 +39,21 @@ foreach_with_prefix scenario {all minimal} {
 
     runto "increment"
 
-    gdb_test "frame" \
+    set re \
 	"#0\\s+callee\\.increment \\(val(=val@entry)?=99\\.0, msg=\\.\\.\\.\\).*"
+    set re_different_entry_val \
+       "#0\\s+callee\\.increment \\(val=99.0, val@entry=.*, msg=\\.\\.\\.\\).*"
+    gdb_test_multiple "frame" "" {
+       -re -wrap $re {
+	   pass $gdb_test_name
+       }
+       -re -wrap $re_different_entry_val {
+	   if { $have_xfail } {
+	      # GCC PR98148
+	      xfail $gdb_test_name
+	   } else {
+	      fail $gdb_test_name
+	   }
+       }
+    }
 }