[3.15] gh-151040: Fix test_c_stack_unwind on RISC-V (GH-152370)

(cherry picked from commit 1b0c05f)

Co-authored-by: Ivy Xu <fakeshadow1337@gmail.com>

Author: ByteFlowing1337

Modules/_testinternalcapi.c

@@ -98,6 +98,11 @@ static const uintptr_t min_frame_pointer_addr = 0x1000;
 // https://refspecs.linuxfoundation.org/ELF/ppc64/PPC-elf64abi-1.9.html#STACK
 #  define FRAME_POINTER_NEXT_OFFSET 0
 #  define FRAME_POINTER_RETURN_OFFSET 2
+#elif defined(__riscv)
+// RISC-V saves the return address at fp[-1], and the previous frame pointer at fp[-2].
+// See: https://riscv-non-isa.github.io/riscv-elf-psabi-doc/#_frame_pointer_convention
+#  define FRAME_POINTER_NEXT_OFFSET -2
+#  define FRAME_POINTER_RETURN_OFFSET -1
 #else
 #  define FRAME_POINTER_NEXT_OFFSET 0
 #  define FRAME_POINTER_RETURN_OFFSET 1