clang/test/CXX/drs/dr1xx.cpp: Add explicit triple x86_64-unknown-unknown to satisfy check7a and check8a since r194240.

For i686 targets and LLP64 targets, we can see;
  error: 'error' diagnostics seen but not expected:
    File clang/test/CXX/drs/dr1xx.cpp Line 761: 'check7a' declared as an array with a negative size
    File clang/test/CXX/drs/dr1xx.cpp Line 765: 'check8a' declared as an array with a negative size
  2 errors generated.