firmware: Fix fw_rw_offset computation in fw_base.S

It seems BFD just does totally nonsensical things for SHN_ABS symbols
when producing position-independent outputs (both -pie and -shared)
for various historical reasons, and so SHN_ABS symbols are still
subject to relocation as far as BFD is concerned (except AArch64,
which fixes it in limited cases that don’t apply here...).

The above affects the _fw_rw_offset provided through fw_base.ldS
linker script which results in OpenSBI firmware failing to boot
when loaded at an address different from FW_TEXT_START.

Fixes: c10e3fe5f9 ("firmware: Add RW section offset in scratch")
Signed-off-by: Jessica Clarke <jrtc27@jrtc27.com>
Reported-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Tested-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Tested-by: Anup Patel <apatel@ventanamicro.com>
Signed-off-by: Anup Patel <apatel@ventanamicro.com>

firmware/fw_base.S

@@ -526,7 +526,7 @@ _link_start:
 _link_end:
 	RISCV_PTR	_fw_reloc_end
 __fw_rw_offset:
-	RISCV_PTR	_fw_rw_offset
+	RISCV_PTR	_fw_rw_start - _fw_start
 
 	.section .entry, "ax", %progbits
 	.align 3

firmware/fw_base.ldS

@@ -64,7 +64,7 @@
 	. = ALIGN(1 << LOG2CEIL((SIZEOF(.rodata) + SIZEOF(.text)
 				+ SIZEOF(.dynsym) + SIZEOF(.rela.dyn))));
 
-	PROVIDE(_fw_rw_offset = (.  - _fw_start));
+	PROVIDE(_fw_rw_start = .);
 
 	/* Beginning of the read-write data sections */