selftest, x86: fix incorrect comment

The current comment indicates it's checking for a 32-bit build
environment, but it actually checks for a 64-bit environment. Fix this.
Signed-off-by: Martin Kelly <martkell@amazon.com>
Acked-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>

tools/testing/selftests/x86/trivial_64bit_program.c

 /*
- * Trivial program to check that we have a valid 32-bit build environment.
+ * Trivial program to check that we have a valid 64-bit build environment.
  * Copyright (c) 2015 Andy Lutomirski
  * GPL v2
  */