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>

CHANGES

@@ -60,6 +60,7 @@
 	Issues resolved (http://bugs.uclibc.org):
 
 	#2353: [lua] fix build with 2010.08-rc1
+	#3751: MIPS: fix BR2_GCC_TARGET_ABI for MIPS n64
 	#4880: New package lcdproc
 	#4886: New package protobuf
 	#5144: Patch to fix ixon bug in uemacs

target/Config.in.arch

@@ -831,7 +831,7 @@ config BR2_GCC_TARGET_ABI
 	default n32		if BR2_MIPS_ABI32
 	default eabi		if BR2_MIPS_EABI
 	default o64		if BR2_MIPS_OABI64
-	default n64		if BR2_MIPS_ABI64
+	default 64		if BR2_MIPS_ABI64
 	default mmixware	if BR2_mmix && BR2_MMIX_ABI_native
 	default gnu		if BR2_mmix && !BR2_MMIX_ABI_native
 	default altivec		if BR2_powerpc && BR2_PPC_ABI_altivec