MIPS: Netlogic: Specify architecture CFLAGS

Use -march=xlr if available, otherwise fallback to mips64. This allows
us to support compilation with MIPS toolchains which are not customized
for XLR.

[ralf@linux-mips.org: And more importantly it works around a gas bug in
binutils 2.21 which otherwise may result in an assertion failure building
arch/mips/kernel/genex.S.  See
http://sourceware.org/bugzilla/show_bug.cgi?id=12915 for details.]

Signed-off-by: Jayachandran C <jayachandranc@netlogicmicro.com>
To: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/2534/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>