DBWALTest.TruncateLastLogAfterRecoverWithoutFlush fails on arm64 and s390x

The platform dependent test `DBWALTest.TruncateLastLogAfterRecoverWithoutFlush` is failing on two non-x86_64 platforms. Both are Ubuntu Xenial and gcc 5.4.0.

* Arm64 - https://github.com/facebook/rocksdb/pull/5932
- CI result - https://travis-ci.org/facebook/rocksdb/jobs/627897529?utm_medium=notification&utm_source=github_status
- Linux kernel - 5.3.0-19-generic
- gcc (Ubuntu/Linaro 5.4.0-6ubuntu1~16.04.11) 5.4.0 20160609

* s390x - https://github.com/facebook/rocksdb/pull/6168
- CI result - https://travis-ci.org/facebook/rocksdb/jobs/628043034?utm_medium=notification&utm_source=github_status
- Linux kernel - 5.3.0-24-generic
- gcc (Ubuntu 5.4.0-6ubuntu1~16.04.12) 5.4.0 20160609

```
[ RUN ] DBWALTest.TruncateLastLogAfterRecoverWithoutFlush
db/db_wal_test.cc:1501: Failure
Expected: (GetAllocatedFileSize(dbname_ + file_before->PathName())) >= (preallocated_size), actual: 512 vs 18427904
terminate called after throwing an instance of 'testing::internal::GoogleTestFailureException'
what(): db/db_wal_test.cc:1501: Failure
Expected: (GetAllocatedFileSize(dbname_ + file_before->PathName())) >= (preallocated_size), actual: 512 vs 18427904
Received signal 6 (Aborted)
#0 /lib/s390x-linux-gnu/libc.so.6(gsignal+0x30) [0x2000c8b81b8] ?? ??:0
#1 /lib/s390x-linux-gnu/libc.so.6(abort+0x14e) [0x2000c8b9726] ?? ??:0
#2 /usr/lib/s390x-linux-gnu/libstdc++.so.6(_ZN9__gnu_cxx27__verbose_terminate_handlerEv+0x19c) [0x2000c5a8e2c] ?? ??:0
#3 /usr/lib/s390x-linux-gnu/libstdc++.so.6(+0xa6776) [0x2000c5a6776] ?? ??:0
#4 /usr/lib/s390x-linux-gnu/libstdc++.so.6(+0xa67d8) [0x2000c5a67d8] ?? ??:0
#5 /usr/lib/s390x-linux-gnu/libstdc++.so.6(__cxa_rethrow+0x64) [0x2000c5a6adc] ?? ??:0
#6 ./db_wal_test(+0x571366) [0x2aa28cf1366] ?? ??:0
#7 ./db_wal_test(+0x567832) [0x2aa28ce7832] ?? ??:0
#8 ./db_wal_test(main+0x38) [0x2aa287d0f10] ?? ??:0
#9 /lib/s390x-linux-gnu/libc.so.6(__libc_start_main+0x10e) [0x2000c8a2ece] ?? ??:0
#10 ./db_wal_test(+0x7f3d4) [0x2aa287ff3d4] ?? ??:0
/bin/bash: line 1: 42668 Aborted (core dumped) ./$t
Makefile:951: recipe for target 'check_some' failed
make: *** [check_some] Error 1
```