[COMMITTED] S390: Fix non-ascii character in fenv.h.

[Florian Weimer]

* Stefan Liebler:

> The comment "isn't" contained a non-ascii character which leads to
> an error if compiled with -finput-charset=ascii:
> error: failure to convert ascii to UTF-8
>
> This is observable in GCC testsuite:
> FAIL: 17_intro/headers/c++1998/charset.cc (test for excess errors)
> FAIL: 17_intro/headers/c++2011/charset.cc (test for excess errors)
> FAIL: 17_intro/headers/c++2014/charset.cc (test for excess errors)
> FAIL: 17_intro/headers/c++2017/charset.cc (test for excess errors)
> FAIL: 17_intro/headers/c++2020/charset.cc (test for excess errors)
>
> Also rewrite the comment above.
>
> Reported-by: Ulrich Weigand <Ulrich.Weigand@de.ibm.com>

Looks good.

I think we should add -finput-charset=ascii to check-install-headers.
Do you want to send a patch for that?