Doxygen fails to parse and render dr_simd_t to HTML

`dr_simd_t` doesn't appear in the current (11-08-23) published HTML at https://dynamorio.org/search?query=dr_simd_t.

The reason Doxygen can't generate a proper HTML definition is because the parser is getting confused by the `ALIGN_VAR()` attribute macro in:
```
typedef union ALIGN_VAR(16) _dr_simd_t {
```
Rendering it as a function `ALIGN_VAR()` rather than a data structure:
```
union ALIGN_VAR (16) _dr_simd_t
```
See `Functions` in https://dynamorio.org/dr__defines_8h.html

Bug exposed by work on https://github.com/DynamoRIO/dynamorio/pull/5835#discussion_r1291516138

Start by reproducing the Doxygen output for the typedef union ALIGN_VAR(16) _dr_simd_t in dr_defines.h and inspect how it is classified in the generated dr__defines_8h.html page. Done means dr_simd_t appears in the published HTML as a data structure rather than under Functions.

Written by the indexing model from the issue text.