gdb: fix sparc build failure of linux-nat

On sparc build failed as:

```
gdb/sparc-linux-nat.c: In member function
  'virtual void sparc_linux_nat_target::fetch_registers(regcache*, int)':
gdb/sparc-linux-nat.c:36:37:
  error: cannot convert 'regcache*' to 'process_stratum_target*'
   36 |   { sparc_fetch_inferior_registers (regcache, regnum); }
      |                                     ^~~~~~~~
      |                                     |
      |                                     regcache*
```

The fix adopts gdb/sparc-nat.h API change in d1e93af6
("gdb: set current thread in sparc_{fetch,collect}_inferior_registers").

gdb/ChangeLog:

	* sparc-linux-nat.c (sparc_linux_nat_target): fix sparc build
	by passing `process_stratum_target*` parameter.