arrow

ARROW-8467: [C++] Fix TestArrayImport tests for big-endian platforms

This PR fixes two problems.

1.  `DecodeMetadata()` always assume that a length of metadata is stored in a little-endian format. According to [this specification](https://github.com/apache/arrow/blob/master/docs/source/format/CDataInterface.rst#the-arrowschema-structure), the length is stored in a native endianness. This causes the following failure. This PR makes the interpretation of the length correct.

```
12: [ RUN      ] TestSchemaImport.Struct
12: /home/ishizaki/Arrow/arrow/cpp/build-support/run-test.sh: line 92: 19528 Segmentation fault      (core dumped) $TEST_EXECUTABLE "$@" 2>&1
12:      19529 Done                    | $ROOT/build-support/asan_symbolize.py
12:      19530 Done                    | ${CXXFILT:-c++filt}
12:      19531 Done                    | $ROOT/build-support/stacktrace_addr2line.pl $TEST_EXECUTABLE
12:      19532 Done                    | $pipe_cmd 2>&1
12:      19533 Done                    | tee $LOGFILE
```

1.  `primitive_buffers_...` used in `TestImportArray` implicitly assumes little-endian. The following is an example.

```
static const uint8_t data_buffer1[] = {1, 2,  3,  4,  5,  6,  7,  8,
                                        9, 10, 11, 12, 13, 14, 15, 16};
static const void* primitive_buffers_no_nulls1[2] = {nullptr, data_buffer1};
...
 TEST_F(TestArrayImport, PrimitiveWithOffset) {
   ...
   FillPrimitive(3, 0, 1, primitive_buffers_no_nulls1);
   CheckImport(ArrayFromJSON(uint16(), "[1027, 1541, 2055]"));
```

The above code works since `03 04 05 06 07 08` is interpreted as `0x0403` (=1027), `0x0605` (=1541), and `0x0807` (=2055) on a little-endian platform. It is interpreted as `0x0304`, `0x0506`, and `0x0708` on a big-endian platform. This causes the following failure. The PR prepares `data_buffer` for each type on both endians.

```
12: [ RUN      ] TestArrayImport.PrimitiveWithOffset
12: /home/ishizaki/Arrow/arrow/cpp/src/arrow/testing/gtest_util.cc:77: Failure
12: Failed
12:
12: @@ -0, +0 @@
12: -1027
12: -1541
12: -2055
12: +772
12: +1286
12: +1800
12: Expected:
12:   [
12:     1027,
12:     1541,
12:     2055
12:   ]
12: Actual:
12:   [
12:     772,
12:     1286,
12:     1800
12:   ]
```

Closes #6958 from kiszk/SPARK-8484

Authored-by: Kazuaki Ishizaki <ishizaki@jp.ibm.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>