[test] env -u is not supported on AIX, use unset instead

Author: Wael Yehia

compiler-rt/test/profile/instrprof-tmpdir.c

@@ -12,7 +12,8 @@
 // RUN: llvm-profdata show ./raw2.profraw | FileCheck %s -check-prefix TMPDIR
 //
 // Check that we fall back to the default path if TMPDIR is missing.
-// RUN: env -u TMPDIR LLVM_PROFILE_FILE="%%t/raw3.profraw" %run %t/binary 2>&1 | FileCheck %s -check-prefix MISSING
+// RUN: %if system-aix %{ unset TMPDIR %}
+// RUN: env %if !system-aix %{ -u TMPDIR %} LLVM_PROFILE_FILE="%%t/raw3.profraw" %run %t/binary 2>&1 | FileCheck %s -check-prefix MISSING
 // RUN: llvm-profdata show ./default.profraw | FileCheck %s -check-prefix TMPDIR
 
 // TMPDIR: Maximum function count: 1