build(blake3): Assume aarch64 to be little-endian on CMake 3.18/3.19 (#1652)

build(blake3): Assume aarch64 to be little-endian on CMake 3.18/3.19 (#1652)

The recently introduced detection of big-endian AArch64 uses the
CMAKE_C_BYTE_ORDER variable, which was introduced in 3.20, but ccache
currently only requires CMake 3.18.

To avoid using an undefined variable, simply assume AArch64 to be
little-endian on CMake versions that don't define the aforementioned
variable. This is okay because big-endian AArch64 has few users and the
subset of them that use CMake 3.18/3.19 is likely negligible.

Reported-by: @awawa-dev
Fixes: https://github.com/ccache/ccache/issues/1651