Makefile: confine $(LIB) to the one platform that needs it, namely irix64

It isn't mentioned anywhere else, and deleting it from the Linux code
path means we don't need to maintain an exhaustive list of 64-bit
architectures.

Makefile

@@ -317,32 +317,12 @@ endif
 # SETUP AND BUILD -- LINUX
 #############################################################################
 
-## Defaults
-LIB=lib
-
 INSTALL=install
 MKDIR=mkdir
 EXTRA_FILES=
 CLIENT_EXTRA_FILES=
 
 ifneq (,$(findstring "$(PLATFORM)", "linux" "gnu_kfreebsd" "kfreebsd-gnu" "gnu"))
-
-  ifeq ($(ARCH),x86_64)
-    LIB=lib64
-  else
-  ifeq ($(ARCH),ppc64)
-    LIB=lib64
-  else
-  ifeq ($(ARCH),s390x)
-    LIB=lib64
-  else
-  ifeq ($(ARCH),aarch64)
-    LIB=lib64
-  endif
-  endif
-  endif
-  endif
-
   BASE_CFLAGS = -Wall -fno-strict-aliasing -Wimplicit -Wstrict-prototypes \
     -pipe -DUSE_ICON
   CLIENT_CFLAGS += $(SDL_CFLAGS)
@@ -824,6 +804,7 @@ else # ifeq netbsd
 #############################################################################
 
 ifeq ($(PLATFORM),irix64)
+  LIB=lib
 
   ARCH=mips