[libc] Improve memcpy for ARM Cortex-M supporting unaligned accesses. (#144872)
This implementation has been compiled with the [pigweed toolchain](https://pigweed.dev/toolchain.html) and tested on:
- Raspberry Pi Pico 2 with the following options\
`--target=armv8m.main-none-eabi`
`-march=armv8m.main+fp+dsp`
`-mcpu=cortex-m33`
- Raspberry Pi Pico with the following options\
`--target=armv6m-none-eabi`
`-march=armv6m`
`-mcpu=cortex-m0+`
They both compile down to a little bit more than 200 bytes and are between 2 and 10 times faster than byte per byte copies.
For best performance the following options can be set in the `libc/config/baremetal/arm/config.json`
```
{
"codegen": {
"LIBC_CONF_KEEP_FRAME_POINTER": {
"value": false
}
},
"general": {
"LIBC_ADD_NULL_CHECKS": {
"value": false
}
}
}
```
llvm-project
7289b678 - [libc] Improve memcpy for ARM Cortex-M supporting unaligned accesses. (#144872)
