[gdb/testsuite] Allow some tests in gdb.base/store.exp to be unsupported

[gdb/testsuite] Allow some tests in gdb.base/store.exp to be unsupported

The test-case gdb.base/store.exp fails with gcc 7.4.0:
...
nr of unexpected failures        27
...

The first FAIL:
...
110       l = add_float (l, r);
(gdb) PASS: gdb.base/store.exp: continue to wack_float
print l
$21 = <optimized out>
FAIL: gdb.base/store.exp: var float l; print old l, expecting -1
...
relates to this bit in the test-case (compiled at -O0):
...
   106  float
   107  wack_float (register float u, register float v)
   108  {
   109    register float l = u, r = v;
   110    l = add_float (l, r);
   111    return l + r;
   112  }
...
and it expects to be able to read and modify variable l before executing line
110, but it already fails to read the value, because l has no DW_AT_location
attribute in the debug info.

Variable l is declared with the register keyword, and GCC implements the
register keyword at -O0 like so:
...
the compiler allocates distinct stack memory for all variables that do not
have the register storage-class specifier; if register is specified, the
variable may have a shorter lifespan than the code would indicate and may
never be placed in memory.
...

The fact that l has no DW_AT_location attribute, matches with the documented
"variable may have a shorter lifespan that code would indicate", (though it
is the most extreme case of it) so the gcc behaviour is valid.  We can of
course improve gcc to generate better debuginfo (filed gcc PR91611), but
this not a wrong-debug problem.

[ The test-case passes with gcc 4.2.1, but for the failing test discussed
above, it passes simply because it doesn't store l in a register. ]

With the debug info missing for l, reading and setting l is unsupported, so
fix the FAIL by marking the test UNSUPPORTED instead.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2019-09-12  Tom de Vries  <tdevries@suse.de>

	* gdb.base/store.exp: Allow register variables to be optimized out at
	-O0.