Fix -Wpointer-sign warning in sol-thread.c

Fix -Wpointer-sign warning in sol-thread.c

This fixes a couple of compiler warnings in rw_common when calling
target_read_memory/target_write_memory due to the type of parameter
"buf" (char *) not matching what these function expect (gdb_byte *).

gdb/ChangeLog:

        * sol-thread.c (rw_common): Change type of parameter "buf"
        to "gdb_byte *".
        (ps_pdwrite, ps_ptwrite): Cast parameter "buf" in call to
        rw_common to "gdb_byte *" instead of "char *".