Use JEMALLOC_ATTR(tls_model("initial-exec)) instead of -ftls-model=initial-exec
so that libjemalloc_pic.a can be directly linked into another library without
needing linker options changes.
Add attributes to malloc, calloc, and posix_memalign, for compatibility with
glibc's declarations.
Add function prototypes for the standard malloc(3) API.