Fix possible out of bound read in armv8_crc32c

[w4123]

The armv8_crc32c function uses ldr to load 4 bytes of data even if the remaining data could be less than 4 bytes. This causes capability bound error.
This error can be triggered, for example, when accessing files in an ext2fs partition, which requires the calculation of crc32.

This piece of code also demonstrates the problem:

#include <stdio.h>
#include <sys/gsb_crc32.h>

int main() {
    unsigned char buf[7];
    printf("%d", armv8_crc32c(0, buf, 7));
}

which prints In-address space security exception (core dumped).

PS: I also wonder if this should be fixed upstream as well.

[bsdjhb]

This does seem like something that should be upstreamed.