compiler: implement recover() built-in function

src/runtime/asm_386.S

@@ -0,0 +1,33 @@
+.section .text.tinygo_scanCurrentStack
+.global tinygo_scanCurrentStack
+.type tinygo_scanCurrentStack, %function
+tinygo_scanCurrentStack:
+    // Sources:
+    //   * https://stackoverflow.com/questions/18024672/what-registers-are-preserved-through-a-linux-x86-64-function-call
+    //   * https://godbolt.org/z/q7e8dn
+
+    // Save callee-saved registers.
+    pushl %ebx
+    pushl %esi
+    pushl %edi
+    pushl %ebp
+
+    // Scan the stack.
+    subl $8, %esp // adjust the stack before the call to maintain 16-byte alignment
+    pushl %esp
+    calll tinygo_scanstack
+
+    // Restore the stack pointer. Registers do not need to be restored as they
+    // were only pushed to be discoverable by the GC.
+    addl $28, %esp
+    retl
+
+
+.section .text.tinygo_longjmp
+.global tinygo_longjmp
+tinygo_longjmp:
+    // Note: the code we jump to assumes eax is set to a non-zero value if we
+    // jump from here.
+    movl 8(%esp), %eax // jumpPC (stash in volatile register)
+    movl 4(%esp), %esp // jumpSP
+    jmpl *%eax