buildroot: fix BR2_GCC_TARGET_ABI for MIPS n64

gcc 4.3/4.4/4.5 accept the following arguments for --with-abi=

"" | 32 | o64 | n32 | 64 | eabi)

So, the "n64" argument coming from buildroot should be changed to "64"
so that gcc's ./configure step does not error out.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
Acked-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>