Rules.mak: MIPS64: Select correct interpreter

Rules.mak: MIPS64: Select correct interpreter

gcc (eg 4.7.3) hardcodes the MIPS64 interpreters like this:
(see gcc/config/linux.h and gcc/config/mips/linux64.h)

o32: UCLIBC_DYNAMIC_LINKER "/lib/ld-uClibc.so.0"
n32: UCLIBC_DYNAMIC_LINKERN32 "/lib32/ld-uClibc.so.0"
n64: UCLIBC_DYNAMIC_LINKER64 "/lib/ld64-uClibc.so.0"

The existing check for MIPS64 in uClibc is wrong because it does
not respect the selected ABI

We fix this by explicitely checking the selected ABI instead of the
selected MIPS variant.

Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Cc: Anthony G. Basile <blueness@gentoo.org>
Signed-off-by: Bernhard Reutner-Fischer <rep.dot.nop@gmail.com>