libc/newlib: fix variables for avr32 headers

Some of the avr32headers related variables are used in different
functions, so have to be declared globally, not locally.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>

 reporter_name="Yann Diorcet <diorcet.yann@gmail.com>"

 reporter_url=""

 reporter_comment="Config to build cross-compiler with mingw as kernel type for

 32 bits Windows target"