all: remove support for LLVM 11 and LLVM 12

This removes a lot of backwards compatibility cruft and makes it
possible to start using features that need LLVM 13 or newer.
For example:

  * https://github.com/tinygo-org/tinygo/pull/2637
  * https://github.com/tinygo-org/tinygo/pull/2830

src/runtime/gc_conservative.go

@@ -233,14 +233,6 @@ func setHeapEnd(newHeapEnd uintptr) {
 // This function can be called again when the heap size increases. The caller is
 // responsible for copying the metadata to the new location.
 func calculateHeapAddresses() {
-	if GOARCH == "wasm" {
-		// This is a workaround for a bug in wasm-ld: wasm-ld doesn't always
-		// align __heap_base and when this memory is shared through an API, it
-		// might result in unaligned memory. For details, see:
-		// https://reviews.llvm.org/D106499
-		// It should be removed once we switch to LLVM 13, where this is fixed.
-		heapStart = align(heapStart)
-	}
 	totalSize := heapEnd - heapStart
 
 	// Allocate some memory to keep 2 bits of information about every block.