Replace JEMALLOC_ATTR with various different macros when it makes sense

Theses newly added macros will be used to implement the equivalent under
MSVC. Also, move the definitions to headers, where they make more sense,
and for some, are even more useful there (e.g. malloc).

include/jemalloc/internal/util.h

@@ -82,8 +82,6 @@
 /******************************************************************************/
 #ifdef JEMALLOC_H_EXTERNS
 
-extern void	(*je_malloc_message)(void *wcbopaque, const char *s);
-
 int	buferror(char *buf, size_t buflen);
 uintmax_t	malloc_strtoumax(const char *nptr, char **endptr, int base);