Add a pages_purge function to wrap madvise(JEMALLOC_MADV_PURGE) calls

This will be used to implement the feature on mingw, which doesn't have
madvise.

include/jemalloc/internal/chunk_mmap.h

@@ -9,6 +9,8 @@
 /******************************************************************************/
 #ifdef JEMALLOC_H_EXTERNS
 
+void	pages_purge(void *addr, size_t length);
+
 void	*chunk_alloc_mmap(size_t size, size_t alignment);
 bool	chunk_dealloc_mmap(void *chunk, size_t size);
 

include/jemalloc/internal/private_namespace.h

@@ -196,6 +196,7 @@
 #define	opt_xmalloc JEMALLOC_N(opt_xmalloc)
 #define	opt_zero JEMALLOC_N(opt_zero)
 #define	p2rz JEMALLOC_N(p2rz)
+#define	pages_purge JEMALLOC_N(pages_purge)
 #define	pow2_ceil JEMALLOC_N(pow2_ceil)
 #define	prof_backtrace JEMALLOC_N(prof_backtrace)
 #define	prof_boot0 JEMALLOC_N(prof_boot0)