[gdb/testsuite] Fix gdb.ada/float-bits.exp for powerpc64le

On powerpc64le-linux, I run into:
...
(gdb) print 16llf#4000921fb54442d18469898cc51701b8#^M
$9 = <invalid float value>^M
(gdb) FAIL: gdb.ada/float-bits.exp: print \
  16llf#4000921fb54442d18469898cc51701b8#
...

The problem is that we're using a hex string for the 128-bit IEEE quad long
double format, but the actual long double float format is:
...
gdbarch_dump: long_double_format = floatformat_ibm_long_double_little^M
...

Fix this by using the hex string obtained by compiling test.c:
...
long double a = 5.0e+25L;
...
like so:
...
$ gcc -mlittle test.c -c -g
...
and running gdb:
...
$ gdb -q -batch test.o -ex "p /x a"
$1 = 0xc1e1c000000000004544adf4b7320335
...
and likewise for -mbig:
...
$ gdb -q -batch test.o -ex "p /x a"
$1 = 0x4544adf4b7320335c1e1c00000000000
...

Tested on powerpc64le-linux.

I excercised the case of floatformat_ibm_long_double_big by
using "set endian big" in the test-case.

Note that for this patch to work correctly, recent commit aaa79cd62b ("[gdb]
Improve printing of float formats") is required.

PR testsuite/29816
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29816
Approved-By: Tom Tromey <tom@tromey.com>

gdb/testsuite/gdb.ada/float-bits.exp

@@ -73,12 +73,40 @@ set gdb_long_double_size [float_size "16llf#0#" ]
 # example, IEEE quad versus i387 long doubles.  Account for that in the
 # tests below.
 
+# Get the used long double format.
+set long_double_format ""
+gdb_test_multiple "maint print architecture" "" {
+    -re "\r\ngdbarch_dump: long_double_format = (\[^\r\n\]*)" {
+	set long_double_format $expect_out(1,string)
+	exp_continue
+    }
+    -re -wrap "" {
+	pass $gdb_test_name
+    }
+}
+
 # Set default values for 128-bit IEEE quad long doubles.
 set valid_long_double "16llf#4000921fb54442d18469898cc51701b8#"
 set printed_long_double "3.1415926535897932384626433832795028"
 set invalid_long_double ""
 set has_invalid_long_double 0
 
+switch -glob $long_double_format {
+    floatformat_ibm_long_double_* {
+	set w1 c1e1c00000000000
+	set w2 4544adf4b7320335
+	switch $long_double_format {
+	    floatformat_ibm_long_double_big {
+		set valid_long_double "16llf#$w2$w1#"
+	    }
+	    floatformat_ibm_long_double_little {
+		set valid_long_double "16llf#$w1$w2#"
+	    }
+	}
+	set printed_long_double "50000000000000000000000000.0"
+    }
+}
+
 if { [istarget x86_64-*-* ] || [istarget i?86-*-*] } {
     # i387 long double have invalid values
     set has_invalid_long_double 1