[PATCH] fw_base.S: Fix assembler error with clang 16+

[Mitchell Horne]

Attempting to build OpenSBI with clang 16 and the following command:

   $ make LLVM=1 PLATFORM=generic

Results in the following error:

    AS        platform/generic/firmware/fw_dynamic.o
   /tmp/fw_dynamic-d000a6.s:429:9: error: symbol '_fw_start' can not be undefined in a subtraction expression
    .dword _fw_rw_start - _fw_start

Work around this issue by eliminating the __fw_rw_offset variable and
performing the offset calculation at run-time instead. This takes
advantage of the fact that the a4 register contains the value of
_fw_start.

Signed-off-by: Mitchell Horne <mhorne at FreeBSD.org>
---
  firmware/fw_base.S | 6 ++----
  1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/firmware/fw_base.S b/firmware/fw_base.S
index b947423..ca34b4c 100644
--- a/firmware/fw_base.S
+++ b/firmware/fw_base.S
@@ -309,8 +309,8 @@ _scratch_init:
  	REG_S	a5, SBI_SCRATCH_FW_SIZE_OFFSET(tp)
   	/* Store R/W section's offset in scratch space */
-	lla	a4, __fw_rw_offset
-	REG_L	a5, 0(a4)
+	lla	a5, _fw_rw_start
+	sub	a5, a5, a4
  	REG_S	a5, SBI_SCRATCH_FW_RW_OFFSET(tp)
   	/* Store fw_heap_offset and fw_heap_size in scratch space */
@@ -536,8 +536,6 @@ _link_start:
  	RISCV_PTR	FW_TEXT_START
  _link_end:
  	RISCV_PTR	_fw_reloc_end
-__fw_rw_offset:
-	RISCV_PTR	_fw_rw_start - _fw_start
   	.section .entry, "ax", %progbits
  	.align 3
-- 
2.41.0