Disable _FILE_OFFSET_BITS if the compiler defaults to it

Disable _FILE_OFFSET_BITS if the compiler defaults to it

Even though building glibc with LFS flags is not supported, and the
the usual way is to patch the build system to avoid it [1]; some system
do enable it by default, and it increases the requirements to build
glibc in such cases (it also does not help newcomers when trying
to build glibc).

The conform namespace and linknamespace tests also do not expect
that flag to be set by default, so disable it as well.

Checked with a build/check for major ABI and some (i386, arm,
mipsel, hppa) with a toolchain that has LFS flags by default.

[1] https://sourceware.org/bugzilla/show_bug.cgi?id=31624
Reviewed-by: DJ Delorie <dj@redhat.com>