[AArch64] Mark single precision pseudo registers unavailable if invalid

[AArch64] Mark single precision pseudo registers unavailable if invalid

I noticed two failure in gdb.trace/mi-trace-frame-collected.exp:

FAIL: gdb.trace/mi-trace-frame-collected.exp: live:
  -trace-frame-collected (register)
FAIL: gdb.trace/mi-trace-frame-collected.exp: tfile:
  -trace-frame-collected (register)

In these cases, we are not collecting registers so the MI command
-trace-frame-collected should only give us the value of the PC.
However, it also gives us all of the single precision pseudo registers,
initialized with 0x0.

We can reproduce this error by simply issuing the
'maint print cooked-register' when no inferior is connected:

~~~
...
(gdb) maint print cooked-register
 Name         Nr  Rel Offset    Size  Type            Cooked value
 x0            0    0      0       8 long            <unavailable>
 x1            1    1      8       8 long            <unavailable>
 ...
 d30         130   62   1540       8 *1              <unavailable>
 d31         131   63   1548       8 *1              <unavailable>
 s0          132   64   1556       4 *1              0x00000000
 s1          133   65   1560       4 *1              0x00000000
 s2          134   66   1564       4 *1              0x00000000
 ...
 s28         160   92   1668       4 *1              0x00000000
 s29         161   93   1672       4 *1              0x00000000
 s30         162   94   1676       4 *1              0x00000000
 s31         163   95   1680       4 *1              0x00000000
 h0          164   96   1684       2 *1              <unavailable>
 h1          165   97   1686       2 *1              <unavailable>
 h2          166   98   1688       2 *1              <unavailable>
 ...
~~~

It turns out GDB does not check if S registers are valid before returning
a value for them.  It should return <unavailable> in this case.

gdb/ChangeLog:

	* aarch64-tdep.c (aarch64_pseudo_read_value): Mark S register as
	unavailable if invalid.