tests/tcg/s390x: Fix the exrl-trt* tests with Clang

tests/tcg/s390x: Fix the exrl-trt* tests with Clang

The exrl-trt* tests use two pre-initialized variables for the
results of the assembly code:

    uint64_t r1 = 0xffffffffffffffffull;
    uint64_t r2 = 0xffffffffffffffffull;

But then the assembly code copies over the full contents
of the register into the output variable, without taking
care of this pre-initialized values:

        "    lgr %[r1],%%r1\n"
        "    lgr %[r2],%%r2\n"

The code then finally compares the register contents to
a value that apparently depends on the pre-initialized values:

    if (r2 != 0xffffffffffffffaaull) {
        write(1, "bad r2\n", 7);
        return 1;
    }

This all works with GCC, since the 0xffffffffffffffff got into
the r2 register there by accident, but it fails completely with
Clang.

Let's fix this by declaring the r1 and r2 variables as proper
register variables instead, so the pre-initialized values get
correctly passed into the inline assembly code.

Message-Id: <20220301092431.1448419-1-thuth@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>