Fix an UBSan complaint introduced by recent change by adding more checks
to word_copy:
```
wasm_interp_fast.c:792:9: runtime error: applying zero offset to null pointer
```
Fix the following warning when loading an aot file without relocations:
```
[20:19:00:528 - 1119F1600]: warning: wasm_runtime_malloc with size zero
```
Fix issues in PR "Refine interp/aot string storage and emitting (#820)",
which had a few issues:
- It looks a wrong byte to mark the flag
- It doesn't work for long strings (>= 0x80 in case of little endian)
This commit fixes them by maintaining a list of loaded symbols while loading
relocation section to avoid reading a string repeatedly, and no need to mark
the flag again.
Fix compile warnings in libc-wasi posix.c:
```
posix.c:880:41: warning: comparison of integers of different signs:
'unsigned long' and 'ssize_t' (aka 'long') [-Wsign-compare]
if (bufoff + iov[i].buf_len < len) {
posix.c:1359:32: warning: comparison of integers of different signs:
'off_t' (aka 'long long') and 'unsigned long long' [-Wsign-compare]
if (ret == 0 && sb.st_size < offset + len)
```
The size used to apply "size = align_uint(size, 4)" may be different
when calculating total size (in get_object_data_sections_size) and
emitting actual data (in aot_emit_object_data_section_info) for the
object data section.
This patch fixes the "Error: emit object data section info failed".
The last argument arg of pthread_create passed from wasm app is opaque
for the library and host, it should be returned back to the wasm app without
any interpretation. There is no point to translate it to a native address.
Actually it harms because the validation seems broken.
