sparc: merge arch/sparc/cpu/*/config.mk to arch/sparc/config.mk

Remove arch/sparc/cpu/{leon2,leon3}/config.mk and move duplicated
defines of -fPIC to arch/sparc/config.mk.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Daniel Hellstrom <daniel@gaisler.com>