runtime, testing: support Goexit, SkipNow, FailNow

Keep a pending Goexit separate from the active panic state so recovered
deferred panics do not cancel the original Goexit unwind. Goexit should
also ignore -panic=trap, since it is not a panic.

Wire testing.FailNow and SkipNow through runtime.Goexit, and run tests
and benchmarks in goroutines. This lets Goexit terminate the user
function without skipping test bookkeeping. Add Goexit/recover coverage
and enable crypto/ecdh in the Linux stdlib test list.

src/runtime/asm_386.S

@@ -43,6 +43,10 @@ tinygo_longjmp:
     // Note: the code we jump to assumes eax is set to a non-zero value if we
     // jump from here.
     movl 4(%esp), %eax
+    movl 8(%eax), %ebx
+    movl 12(%eax), %esi
+    movl 16(%eax), %edi
+    movl 20(%eax), %ebp
     movl 0(%eax), %esp // jumpSP
     movl 4(%eax), %eax // jumpPC (stash in volatile register)
     jmpl *%eax