riscv-unknown-elf-as wont accept .section

[Paul via D.gnu]

I'm following a tutorial that offers this file (start.s),
    .section ".text.boot"

    .globl _start
    _start:
       la sp, _stack_start
       call dstart
    _hlt:
       j _hlt  # jump to _hlt

...assembled with this,
**riscv64-unknown-elf-as  -mno-relax -march=rv64imac start.s -c 
-o start.o**

...which responds with,
**start.s: Assembler messages:**
**start.s:1: Error: unrecognized opcode `.section ".text.boot"'**

The only way I can get start.s to compile is by removing these 
first three lines:

    .section ".text.boot"

    .globl _start


Any help would be appreciated.