[lldb] support ieee_single and ieee_double gdbtypes for registers (#150268)
Some gdb remote servers send target.xml that contains
```
<reg name='ft0' bitsize='32' type='ieee_single' dwarf_regnum='32'/>
<reg name='ft1' bitsize='32' type='ieee_single' dwarf_regnum='33'/>
<reg name='ft2' bitsize='32' type='ieee_single' dwarf_regnum='34'/>
<reg name='ft3' bitsize='32' type='ieee_single' dwarf_regnum='35'/>
<reg name='ft4' bitsize='32' type='ieee_single' dwarf_regnum='36'/>
<reg name='ft5' bitsize='32' type='ieee_single' dwarf_regnum='37'/>
<reg name='ft6' bitsize='32' type='ieee_single' dwarf_regnum='38'/>
<reg name='ft7' bitsize='32' type='ieee_single' dwarf_regnum='39'/>
```
it seems like a valid and supported type in gdb.
from gdb16.3/gdb/target_descriptions.c (could not find a way to link
it).
```
case TDESC_TYPE_IEEE_SINGLE:
m_type = init_float_type (alloc, -1, "builtin_type_ieee_single",
floatformats_ieee_single);
return;
case TDESC_TYPE_IEEE_DOUBLE:
m_type = init_float_type (alloc, -1, "builtin_type_ieee_double",
floatformats_ieee_double);
return;
```
### Testplan
updated unittest to test this.
Reviewers: @clayborg , @jeffreytan81 , @Jlalond
ea480cc665