Make the malloc(3) family of functions weak and make their non-weak

Make the malloc(3) family of functions weak and make their non-weak
implementations visible for use by applications. The functions $F that
are now weak symbols are:
	allocm, calloc, dallocm, free, malloc, malloc_usable_size,
	nallocm, posix_memalign, rallocm, realloc, sallocm

The non-weak implementations of $F are exported as __$F.

Submitted by:	stevek@juniper.net
Reviewed by:	jasone@, kib@
Approved by:	jasone@ (jemalloc)
Obtained from:	juniper Networks, Inc