kvm-unit-tests: fix build on x86-64 with specific gcc versions

kvm-unit-tests: fix build on x86-64 with specific gcc versions

kvm-unit-tests uses the following code on x86/x86-64:

static inline u64 scale_delta(u64 delta, u64 mul_frac)
{
        u64 product, unused;

        __asm__ (
                "mul %3"
                : "=d" (product), "=a" (unused) : "1" (delta), "rm" ((u64)mul_frac) );

        return product;
}

The "mul" instruction does not have a suffix that indicates the width of
the data being multiplied. When the data is passed in a register, there
is no need to specify the width, but with some gcc versions, the data is
passed as a memory reference, and therefore the assembler does not know
the width of the data to be multiplied. It causes the following build
failure:

x86/hyperv_clock.c: Assembler messages:
x86/hyperv_clock.c:21: Error: no instruction mnemonic suffix given and no register operands; can't size instruction

Since the data being multiplied is 64 bit, we explicitly specify the
instruction as being "mulq".

Fixes:

  http://autobuild.buildroot.net/results/a4a65d01f049db83a93de92660f228dd18532625/

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>