Fix segfault with the help of gdb via Nix

Makefile

@@ -1,5 +1,7 @@
+CC=riscv64-unknown-elf-gcc
 AS=riscv64-unknown-elf-as
 ASFLAGS=-g -mabi=ilp32e -march=rv32ec
+CFLAGS=$(ASFLAGS)
 LD=riscv64-unknown-elf-ld
 
 all: calc.elf

calc.s

@@ -108,7 +108,11 @@ regdump_loop:
     la a0, buf          # load address of buf into a0 as dest
     li a2, 2            # copy 2 bytes
 
+    addi sp, sp, -4  # allocate stack space
+    sw ra, 0(sp)     # save contents of ra
     jal memcpy
+    lw ra, 0(sp)     # save contents of ra
+    addi sp, sp, 4   # deallocate stack space
     # append ': \n'
     li t0, ':
     sb t0, 0(a0)

cmain.c

@@ -0,0 +1,5 @@
+extern void regdump(void);
+
+void cmain(void) {
+  regdump();
+}

shell.nix

@@ -0,0 +1,12 @@
+let
+  nixpkgs = fetchTarball "https://github.com/NixOS/nixpkgs/tarball/release-24.11";
+  pkgs = (import nixpkgs {}).pkgsCross.riscv32-embedded;
+in
+
+# callPackage is needed due to https://github.com/NixOS/nixpkgs/pull/126844
+pkgs.pkgsStatic.callPackage ({ mkShell, zlib, pkg-config, file, gdb }: mkShell {
+  # these tools run on the build platform, but are configured to target the host platform
+  nativeBuildInputs = [ pkg-config file gdb ];
+  # libraries needed for the host platform
+  buildInputs = [ zlib ];
+}) {}