[gdb/tdep] Fix mmap syscall mapping

There are a few spots where an mmap system call is mapped onto enum
gdb_syscall value gdb_sys_mmap2.

Strictly speaking, this is incorrect.

Fix this by mapping to enum gdb_syscall value gdb_old_mmap instead.

No functional changes: both gdb_old_mmap and gdb_sys_mmap2 are handled the
same in record_linux_system_call.

Tested by rebuilding on x86_64-linux.