arm64: add endianness option to LDFLAGS instead of LD

With the recent syntax extension, Kconfig is now able to evaluate the
compiler / toolchain capability.

However, accumulating flags to 'LD' is not compatible with the way
it works; 'LD' must be passed to Kconfig to call $(ld-option,...)
from Kconfig files.  If you tweak 'LD' in arch Makefile depending on
CONFIG_CPU_BIG_ENDIAN, this would end up with circular dependency
between Makefile and Kconfig.

Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Change-Id: I8a7654684975d45e979917e3b1c4b6249dec02ec

arch/arm64/Makefile

@@ -91,12 +91,12 @@ KBUILD_AFLAGS	+= $(lseinstr) $(vdso32)
 ifeq ($(CONFIG_CPU_BIG_ENDIAN), y)
 KBUILD_CPPFLAGS	+= -mbig-endian
 AS		+= -EB
-LD		+= -EB
+LDFLAGS		+= -EB -maarch64elfb
 UTS_MACHINE	:= aarch64_be
 else
 KBUILD_CPPFLAGS	+= -mlittle-endian
 AS		+= -EL
-LD		+= -EL
+LDFLAGS		+= -EL -maarch64elf
 UTS_MACHINE	:= aarch64
 endif