runtime: lock output in print/println

This ensures that calls to print/println happening in different threads
are not interleaved. It's a task.PMutex, so this should only change
things when threading is used.

This matches the Go compiler, which does the same thing:
https://godbolt.org/z/na5KzE7en

The locks are not recursive, which means that we need to be careful to
not call `print` or `println` inside a runtime.print* implementation,
inside putchar (recursively), and inside signal handlers. Making them
recursive might be useful to do in the future, but it's not really
necessary.

compiler/testdata/defer-cortex-m-qemu.ll

@@ -122,12 +122,18 @@ declare void @runtime.destroyDeferFrame(ptr dereferenceable_or_null(24), ptr) #2
 ; Function Attrs: nounwind
 define internal void @"main.deferSimple$1"(ptr %context) unnamed_addr #1 {
 entry:
+  call void @runtime.printlock(ptr undef) #4
   call void @runtime.printint32(i32 3, ptr undef) #4
+  call void @runtime.printunlock(ptr undef) #4
   ret void
 }
 
+declare void @runtime.printlock(ptr) #2
+
 declare void @runtime.printint32(i32, ptr) #2
 
+declare void @runtime.printunlock(ptr) #2
+
 ; Function Attrs: nounwind
 define hidden void @main.deferMultiple(ptr %context) unnamed_addr #1 {
 entry:
@@ -250,14 +256,18 @@ rundefers.end7:                                   ; preds = %rundefers.loophead1
 ; Function Attrs: nounwind
 define internal void @"main.deferMultiple$1"(ptr %context) unnamed_addr #1 {
 entry:
+  call void @runtime.printlock(ptr undef) #4
   call void @runtime.printint32(i32 3, ptr undef) #4
+  call void @runtime.printunlock(ptr undef) #4
   ret void
 }
 
 ; Function Attrs: nounwind
 define internal void @"main.deferMultiple$2"(ptr %context) unnamed_addr #1 {
 entry:
+  call void @runtime.printlock(ptr undef) #4
   call void @runtime.printint32(i32 5, ptr undef) #4
+  call void @runtime.printunlock(ptr undef) #4
   ret void
 }