mk: Fix compiling jemalloc for powerpc64

We forgot to pass down the `-m64` flag to gcc, so we were actually compiling
powerpc code which would then later fail to link!

Author: alexcrichton

mk/cfg/powerpc64-unknown-linux-gnu.mk

@@ -8,6 +8,7 @@ CFG_LIB_NAME_powerpc64-unknown-linux-gnu=lib$(1).so
 CFG_STATIC_LIB_NAME_powerpc64-unknown-linux-gnu=lib$(1).a
 CFG_LIB_GLOB_powerpc64-unknown-linux-gnu=lib$(1)-*.so
 CFG_LIB_DSYM_GLOB_powerpc64-unknown-linux-gnu=lib$(1)-*.dylib.dSYM
+CFG_JEMALLOC_CFLAGS_powerpc64-unknown-linux-gnu := -m64
 CFG_CFLAGS_powerpc64-unknown-linux-gnu := -m64 $(CFLAGS)
 CFG_GCCISH_CFLAGS_powerpc64-unknown-linux-gnu := -Wall -Werror -g -fPIC -m64 $(CFLAGS)
 CFG_GCCISH_CXXFLAGS_powerpc64-unknown-linux-gnu := -fno-rtti $(CXXFLAGS)