Remove explicit mips64 -mabi parameter

This breaks when building for n32 ABI on mips64.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
---

diff -Nuar Botan-1.10.8-orig/src/build-data/cc/gcc.txt Botan-1.10.8/src/build-data/cc/gcc.txt
--- Botan-1.10.8-orig/src/build-data/cc/gcc.txt	2014-04-10 17:11:44.000000000 +0300
+++ Botan-1.10.8/src/build-data/cc/gcc.txt	2014-10-23 09:20:34.506722323 +0300
@@ -92,7 +92,6 @@
 # The 'linking' bit means "use this for both compiling *and* linking"
 <mach_abi_linking>
 x86_64  -> "-m64"
-mips64  -> "-mabi=64"
 s390    -> "-m31"
 s390x   -> "-m64"
 sparc32 -> "-m32 -mno-app-regs"