linux-user/riscv: Adjust vdso signal frame cfa offsets

A typo in sizeof_reg put the registers at the wrong offset.

Simplify the expressions to use positive addresses from the
start of uc_mcontext instead of negative addresses from the
end of uc_mcontext.

Reported-by: Vineet Gupta <vineetg@rivosinc.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
(cherry picked from commit 1b21fe27e7)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

linux-user/riscv/vdso.S

@@ -101,12 +101,12 @@ endf __vdso_flush_icache
 	.cfi_startproc simple
 	.cfi_signal_frame
 
-#define sizeof_reg	(__riscv_xlen / 4)
+#define sizeof_reg	(__riscv_xlen / 8)
 #define sizeof_freg	8
-#define B_GR	(offsetof_uc_mcontext - sizeof_rt_sigframe)
-#define B_FR	(offsetof_uc_mcontext - sizeof_rt_sigframe + offsetof_freg0)
+#define B_GR	0
+#define B_FR	offsetof_freg0
 
-	.cfi_def_cfa	2, sizeof_rt_sigframe
+	.cfi_def_cfa	2, offsetof_uc_mcontext
 
 	/* Return address */
 	.cfi_return_column 64