[LIT]Accept cat_64 command name on AIX in shtest

[jsji]

Summary

AIX may use cat_64 for 64 bit cat, this is just update the lit test to accept the name as well.

[shchenz]

LGTM.

[daltenty]

Seems fine, just noting as well that cat_64 is the GNU core utils version as well, not just a 64-bit version of the system one.

llvm/utils/lit/tests/shtest-format.py

 # CHECK: FAIL: shtest-format :: external_shell/fail.txt
 # CHECK-NEXT: *** TEST 'shtest-format :: external_shell/fail.txt' FAILED ***
 # CHECK: Command Output (stdout):
 # CHECK-NEXT: --
 # CHECK-NEXT: line 1: failed test output on stdout
 # CHECK-NEXT: line 2: failed test output on stdout
 # CHECK: Command Output (stderr):
 # CHECK-NEXT: --
-# CHECK-NEXT: cat{{(\.exe)?}}: {{cannot open does-not-exist|does-not-exist: No such file or directory}}
+# CHECK-NEXT: cat{{(_64)?(\.exe)?}}: {{cannot open does-not-exist|does-not-exist: No such file or directory}}
 # CHECK: --
 
 # CHECK: FAIL: shtest-format :: external_shell/fail_with_bad_encoding.txt
 # CHECK-NEXT: *** TEST 'shtest-format :: external_shell/fail_with_bad_encoding.txt' FAILED ***
 # CHECK: Command Output (stdout):
 # CHECK-NEXT: --
 # CHECK-NEXT: a line with bad encoding:
 # CHECK: --