gh-151040: Fix test_c_stack_unwind on RISC-V (#152370)

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
 #elif defined(__loongarch__)
 // On LoongArch, the frame pointer is the caller's stack pointer.
 // The saved frame pointer is stored at fp[-2], and the return