parser: fix build on solaris

`FS` is defined in `regset.h` on solaris and smartos.

BUG=
R=ulan@chromium.org, danno@chromium.org

Review URL: https://codereview.chromium.org/176403004

Patch from Fedor Indutny <fedor.indutny@gmail.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19602 ce2b1a6d-e550-0410-aec6-3dcde31c8c00