Support dynamic dual/quad loop selection on aarch64

Support dynamic dual/quad loop selection on aarch64

Currently, there are three variants available on aarch64, qdf24xx, thunderx2t99, and cortex-a72.
-DCLIB_N_PREFETCHES is passed to source code to select dual/quad implementation.
Besides, different compiler options are applied on these critical functions.

gcc-7.3.0 reports ICE(internal compiler error) with -mtune=thunderx2t99,
so -mtune=thunderx2t99 is enabled only when gcc version is greater than 7.3.0

Cavium ThunderX2, Impermenter 0x43, Part 0x0af
    -march=armv8-a+crc+crypto -mtune=thunderx2t99
Qualcomm Centriq 2400, Impermenter 0x51, Part 0xc00
    -march=armv8.1-a+crc+crypto -mtune=qdf24xx
Cortex-A72, Impermenter 0x41, Part 0xd08
    -march=armv8-a+crc+crypto -mtune=cortex-a72

Change-Id: Id5649c6325c1e642d0fd42535e3908793b13e02a
Signed-off-by: Lijian Zhang <Lijian.Zhang@arm.com>
Reviewed-by: Sirshak Das <sirshak.das@arm.com>
Reviewed-by: Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>