[perl #50996] [BUG] gc bug with perl6grammar in pdd17pmc branch

[Allison Randal via RT]

Resolved in r26278. It was an alignment problem in the parrot_string_t
struct, triggered by 'Parrot_unmake_COW', when it does a 'memcpy' of the
string's original buffer to the copy's buffer. Fixed by moving the
UINTVAL 'bufused' after the char* 'strstart'.