arch-arm: Fix aapcs32/aapcs64 compilation issues

arch-arm: Fix aapcs32/aapcs64 compilation issues

Some compilers won't build ARM due to how guest ABI
has been implemented.

The error is: "left shift count >= width of type"
[-Werror=shift-count-overflow]

The error is triggered when there is a left shift > the variable size
(in bits); this leads to undefined behaviour.

This is a compile time vs run time problem; the code is technically
fine, but the compiler is not able to understand this.

For example in aapcs64:

struct Argument<Aapcs64, Integer, typename std::enable_if<
 std::is_integral<Integer>::value>::type> : public Aapcs64ArgumentBase
{
    [...]
    if (sizeof(Integer) == 16 && state.ngrn + 1 <= state.MAX_GRN) {
        Integer low = tc->readIntReg(state.ngrn++);
        Integer high = tc->readIntReg(state.ngrn++);
        high = high << 64;
        return high | low;
    }
}

Even if the sizeof operator will be evaluated at compile time,
the block will be executed at runtime: the block will still be part of
the code if Integer = uint32_t.
The compiler will then throw an error because we are left shifting an
uint32_t by 64 bits.

Error arising on:
Compiler: gcc/5.4.0
Distro: Ubuntu 16.04 LTS

Change-Id: Iaafe030b7262c5fb162afe7118ae592a1a759a58
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/26990
Reviewed-by: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>