Configuring newlib sbrk

Hullo everyone. I am doing a build of newlib that targets a RISC-V. I
currently have this config line

./configure --prefix=/xxx/riscv-gnu-toolchain/riscv
--target=riscv32-unknown-elf --with-arch=rv32imaf --with-abi=ilp32f
--disable-linux

Currently I have my own _sbrk stub function configured. However as the
system has got more functional it is now using an axi shared driver and ram
is memory mapped. So I am wondering if I really need to set up my own _sbrk
stub at all? I would rather rely on the Newlib entirely. Can I tell the
configure that I want to use a single threaded implementation of the
sbrk/malloc/free and associated functions? Can I also specify where the
heap_start/heap_end/stack_start is to be found?
-- 
Damien Towning