bjam' always 'bus error' on NetBSD/sparc (presumably, other strict-alignement 32 bits arch as well)

Originally reported downstream at pkgsrc (http://gnats.netbsd.org/55269).

# The observation:

Trying to compile the "boost" meta-package in pkgsrc on NetBSD 9.0/sparc, 'bjam' always crashes with a 'bus error'. The culprit is the function `timestamp_init` in `tools/build/src/engine/timestamp.cpp`, which does:

`time->secs = secs;`

where `secs` are `time_t`, currently 8 bytes. This is only legal on SPARC if `time->secs` is 8-bytes aligned, which it isn't, so the 8-bytes store causes a 'bus error'. The reason for the nonalignment is that it comes from the hash table implementation in hash.cpp, which does:

`#define hash_item_data(item) ((HASHDATA *)((char *)item + sizeof(ITEM)))`

`ITEM` is the size of a pointer, and `item` is properly aligned (at least 8 bytes), so this is _never_ aligned properly with 32-bits pointers... This result in reliable crashes on 32-bits machine with strict alignment requirements (i.e., sparc).

# my fix

The following patch fixes the problem for me, but might be a bit overkill (memory wastage) on 32-bits architecture that supports unaligned access. It should be a no-op on 64-bits architecture.

```
--- tools/build/src/engine/hash.cpp.orig 2019-12-10 01:20:17.000000000 +0100
+++ tools/build/src/engine/hash.cpp 2020-05-16 14:55:46.086210410 +0200
@@ -33,8 +33,8 @@
typedef struct item ITEM;
struct item
{
- ITEM * next;
-};
+ ITEM * next __attribute__ ((aligned (8)));
+} __attribute__ ((aligned (8)));

#define MAX_LISTS 32
```
There is probably better solution than that, that would generally respect alinement's rules on all platforms.

Start with tools/build/src/engine/timestamp.cpp and tools/build/src/engine/hash.cpp, especially timestamp_init and the hash_item_data layout described in the report. Reproduce the bjam bus error on NetBSD/sparc or another strict-alignment 32-bit platform, then verify that the alignment handling is portable and the build completes without the crash.

Written by the indexing model from the issue text.