[libc] Add support for string/memory_utils functions for AArch64 without HW FP/SIMD (#137592)
Add conditional compilation to add support for AArch64 without vector
registers and/or hardware FPUs by using the generic implementation.
**Context:**
A few functions were hard-coded to use vector registers/hardware FPUs.
This meant that libc would not compile on architectures that did not
support these features. This fix falls back on the generic
implementation if a feature is not supported.