Doxygen fails to parse and render dr_simd_t to HTML

[AssadHashmi]

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 #5835 (comment)