statfs: fix compile error when UCLIBC_LINUX_SPECIFIC is not set

statfs() is a Linux-specific function.  When building without
UCLIBC_LINUX_SPECIFIC set, libc_hidden_proto(statfs) in
include/sys/statfs.h is unmatched with libc_hidden_def(statfs)
in libc/sysdeps/linux/common/statfs.c, leading to a compile
error.  This patch fixes this, as well as adds statfs() to the
list of Linux-specific functions in extra/Configs/Config.in.

Signed-off-by: Anthony G. Basile <blueness@gentoo.org>
Signed-off-by: Bernhard Reutner-Fischer <rep.dot.nop@gmail.com>
Config.in.arch: enable long double math for xtensa

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Bernhard Reutner-Fischer <rep.dot.nop@gmail.com>