Fix AIX gdb build with C++ compiler

Fix AIX gdb build with C++ compiler

We currently get:

 ../../src/gdb/aix-thread.c: In function 'int pdc_read_data(pthdb_user_t, void*, pthdb_addr_t, size_t)':
 ../../src/gdb/aix-thread.c:465:46: error: invalid conversion from 'void*' to 'gdb_byte* {aka unsigned char*}' [-fpermissive]
    status = target_read_memory (addr, buf, len);
					       ^

 ../../src/gdb/aix-thread.c: In function 'void aix_thread_resume(target_ops*, ptid_t, int, gdb_signal)':
 ../../src/gdb/aix-thread.c:1010:46: error: invalid conversion from 'void*' to 'int*' [-fpermissive]
	 gdb_signal_to_host (sig), (void *) tid);
					       ^
 ../../src/gdb/aix-thread.c:243:1: error:   initializing argument 5 of 'int ptrace64aix(int, int, long long int, int, int*)' [-fpermissive]
  ptrace64aix (int req, int id, long long addr, int data, int *buf)

 ../../src/gdb/rs6000-nat.c: In function 'gdb_byte* rs6000_ptrace_ldinfo(ptid_t)':
 ../../src/gdb/rs6000-nat.c:596:36: error: invalid conversion from 'void*' to 'gdb_byte* {aka unsigned char*}' [-fpermissive]
    gdb_byte *ldi = xmalloc (ldi_size);
				     ^
 ../../src/gdb/rs6000-nat.c:615:36: error: invalid conversion from 'void*' to 'gdb_byte* {aka unsigned char*}' [-fpermissive]
	ldi = xrealloc (ldi, ldi_size);
				     ^

(and more instances of the same).

gdb/ChangeLog:
2016-04-21  Pedro Alves  <palves@redhat.com>

	* aix-thread.c (pdc_read_data, pdc_write_data): Add cast.
	(aix_thread_resume): Use PTRACE_TYPE_ARG5.
	* rs6000-nat.c (rs6000_ptrace64): Use PTRACE_TYPE_ARG5.
	(rs6000_ptrace_ldinfo): Change type of 'ldi' local to void
	pointer, and cast return to gdb_byte pointer.