gdb/testsuite: Add support for LoongArch in gdb.base/float.exp

gdb/testsuite: Add support for LoongArch in gdb.base/float.exp

The test results on LoongArch as follows:

Without this patch:

```
$ make check-gdb TESTS="gdb.base/float.exp"
=== gdb Summary ===

 # of expected passes		2
 # of unexpected failures	1

```
With this patch:

```
$ make check-gdb TESTS="gdb.base/float.exp"
=== gdb Summary ===

 # of expected passes		3

```

Signed-off-by: Hui Li <lihui@loongson.cn>
Reviewed-By: Tom Tromey <tom@tromey.com>
Approved-By: Simon Marchi <simon.marchi@efficios.com>
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>