UNREG: mark SRT as writable in generated C code

Noticed section mismatch on UNREG build failure:

```
  HC [stage 1] libraries/integer-gmp/dist-install/build/GHC/Integer/Type.o

     error: conflicting types for 'ufu0_srt'
     static StgWord ufu0_srt[]__attribute__((aligned(8)))= {
                    ^~~~~~~~

     note: previous declaration of 'ufu0_srt' was here
     IRO_(ufu0_srt);
          ^~~~~~~~
```

`IRO_` is a 'const' qualifier.

The error is a leftover from commit 838b6903
"Merge FUN_STATIC closure with its SRT" where part of SRT was moved
into closure itself and made SRTs writable.

This change puts all SRTs into writable section.

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>

Reviewers: simonmar, bgamari

Subscribers: rwbarton, thomie, carter

Differential Revision: https://phabricator.haskell.org/D4731