Define TARGET_IS_64_BIT when building libdebuggerd_client for android64.

This is needed to connect to the right debuggerd server for the crashing
process. Without this change, the following errors are observed, because a
32-bit process is trying to connect to the 64-bit debuggerd (as opposed to
debuggerd32):

  Fatal signal 11 (SIGSEGV), code 1, fault addr 0x0 in tid 14153 (static_crasher)
  debuggerd: handling request: pid=14153 uid=0 gid=0 tid=14153
  debuggerd: Not allowed to redirect action 0 to 32 bit debuggerd

Bug: 24414818
Change-Id: I2f275fbc804b0abc6e876a743e51dd8494817103