Fix mis-aligned access issues with s390x (#4702)

This fixes two problems: minimum symbol alignment for the LARL
instruction, and alignment requirements for LRL/LGRL etc.

The first problem is that the LARL instruction used to load a
symbol address (PC relative) requires that the target symbol
is at least 2-byte aligned.  This is always guaranteed for code
symbols (all instructions must be 2-aligned anyway), but not
necessarily for data symbols.

Other s390x compilers fix this problem by ensuring that all
global symbols are always emitted with a minimum 2-byte
alignment.  This patch introduces an equivalent mechanism
for cranelift:
- Add a symbol_alignment routine to TargetIsa, similar to the
  existing code_section_alignment routine.
- Respect symbol_alignment as minimum alignment for all symbols
  emitted in the object backend (code and data).

The second problem is that PC-relative instructions that
directly *access* data (like LRL/LGRL, STRL/STGRL etc.)
not only have the 2-byte requirement like LARL, but actually
require that their memory operand is *naturally* aligned
(i.e. alignment is at least the size of the access).

This property (natural alignment for memory accesses) is
supposed to be provided by the "aligned" flag in MemFlags;
however, this is not implemented correctly at the moment.

To fix this, this patch:
- Only emits PC-relative memory access instructions if the
  "aligned" flag is set in the associated MemFlags.
- Fixes a bug in emit_small_memory_copy and emit_small_memset
  which currently set the aligned flag unconditionally, ignoring
  the actual alignment info passed by their caller.

Tested with wasmtime and cg_clif.

cranelift/frontend/src/frontend.rs

@@ -816,7 +816,9 @@ impl<'a> FunctionBuilder<'a> {
             return;
         }
 
-        flags.set_aligned();
+        if u64::from(src_align) >= access_size && u64::from(dest_align) >= access_size {
+            flags.set_aligned();
+        }
 
         // Load all of the memory first. This is necessary in case `dest` overlaps.
         // It can also improve performance a bit.
@@ -903,7 +905,9 @@ impl<'a> FunctionBuilder<'a> {
             let size = self.ins().iconst(config.pointer_type(), size as i64);
             self.call_memset(config, buffer, ch, size);
         } else {
-            flags.set_aligned();
+            if u64::from(buffer_align) >= access_size {
+                flags.set_aligned();
+            }
 
             let ch = u64::from(ch);
             let raw_value = if int_type == types::I64 {