UNREG: don't prefix asm prefixes in via-C mode

commit 64c54fff
("Mark system and internal symbols as private symbols in asm")

Added `internalNamePrefix` helper. Unfortunately it
generates invalid label in unregisterised mode:

```
$ ./configure --enable-unregisterised
/tmp/ghc19372_0/ghc_4.hc:2831:22: error:
     error: expected identifier or '(' before '.' token
     static const StgWord .Lcl3_info[]__attribute__((aligned(8)))= {
                          ^
```

Here asm-style prefix is applied to C symbol.
The fix is simple: apply asm-style labels only to assembly code.
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>

Reviewers: simonmar, last_g, bgamari

Reviewed By: simonmar

Subscribers: rwbarton, carter

Differential Revision: https://phabricator.haskell.org/D5207

compiler/cmm/CLabel.hs

@@ -1339,7 +1339,7 @@ instance Outputable ForeignLabelSource where
 
 internalNamePrefix :: Name -> SDoc
 internalNamePrefix name = getPprStyle $ \ sty ->
-  if codeStyle sty && isRandomGenerated then
+  if asmStyle sty && isRandomGenerated then
     sdocWithPlatform $ \platform ->
       ptext (asmTempLabelPrefix platform)
   else