selftests: harness: Rename is_signed_type() to avoid collision with overflow.h

Rename is_signed_type() to is_signed_var() to avoid colliding with a macro
of the same name defined by tools' linux/overflow.h.  This fixes warnings
(and presumably potential test failures) in tests that utilize the
selftests harness and happen to (indirectly) include overflow.h.

  In file included from tools/include/linux/bits.h:34,
                   from tools/include/linux/bitops.h:14,
                   from tools/include/linux/hashtable.h:13,
                   from include/kvm_util.h:11,
                   from x86/userspace_msr_exit_test.c:11:
  tools/include/linux/overflow.h:31:9: error: "is_signed_type" redefined [-Werror]
     31 | #define is_signed_type(type)       (((type)(-1)) < (type)1)
        |         ^~~~~~~~~~~~~~
  In file included from include/kvm_test_harness.h:11,
                   from x86/userspace_msr_exit_test.c:9:
  ../kselftest_harness.h:754:9: note: this is the location of the previous definition
    754 | #define is_signed_type(var)       (!!(((__typeof__(var))(-1)) < (__typeof__(var))1))
        |         ^~~~~~~~~~~~~~

Use a separate definition, at least for now, as many selftests build
without tools/include in their include path.

Fixes: fc920999 ("tools headers: Synchronize linux/bits.h with the kernel sources")
Cc: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20250624231930.583689-1-seanjc@google.com


Signed-off-by: Sean Christopherson <seanjc@google.com>