arch/riscv: Move _start to the beginning of the bootblock

The different entry points (0x100, 0x140, ...), which were defined in
the RISC-V Privileged Specification 1.7, aren't used anymore. Instead
the Spike bootrom jumps at the start of our image, and traps are handled
through mtvec.

Change-Id: I865adec5e7a752a25bac93a45654ac06e27d5a8e
Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Reviewed-on: https://review.coreboot.org/15283
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>

src/arch/riscv/bootblock.S

@@ -17,20 +17,7 @@
 #include <arch/encoding.h>
 
 .section ".text._start", "ax", %progbits
-// Maybe there's a better way.
-# machine mode handler when in supervisor mode
-.space 0x140
-supervisor_machine_handler:
-  j supervisor_trap_entry
 
-# handler for when
-.space 0x7c
-.globl machine_handler
-machine_handler:
-#  call trap_handler
-  j trap_entry
-
-.space 0x3c
 .globl _start
 _start: