Use gcc -finput-charset=ascii for check-installed-headers.

A non-ascii character in the installed headers leads now to:
error: failure to convert ascii to UTF-8

Such a finding in s390 specific fenv.h leads to fails in GCC testsuite.
See glibc commit 08aea89e.

Adding this gcc option also to our tests was proposed by Florian Weimer.

This change also found a hit in resource.h where now "microseconds" is used.
I've adjusted all the resource.h files.

I've used the following command to check for further hits in headers.
LC_ALL=C find -name "*.h" -exec grep -PHn "[\x80-\xFF]" {} \;

Tested on s390x and x86_64.

Reviewed-by: Zack Weinberg <zackw@panix.com>