arch/arm64/Makefile.mk: Switch linker to GNU GCC

TF-A migrates the default choice of linker to GCC in order to enable
LTO. Change BL31_LDFLAGS from `--emit-relocs` to '-Wl,--emit-relocs', so
that GCC is able to pass `--emit-relocs` to the linker.

[1]: https://review.trustedfirmware.org/c/26703

BUG=b:338420310
TEST=emerge-geralt coreboot
TEST=./util/abuild/abuild -t google/geralt -b geralt -a
TEST=./util/abuild/abuild -t google/oak -b elm -a

Change-Id: I65b96aaa052138592a0f57230e1140a1bb2f07ac
Signed-off-by: Yidi Lin <yidilin@chromium.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/82189
Reviewed-by: Eric Lai <ericllai@google.com>
Reviewed-by: Yu-Ping Wu <yupingso@google.com>
Reviewed-by: Julius Werner <jwerner@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/arch/arm64/Makefile.mk

@@ -173,14 +173,13 @@ BL31_MAKEARGS += IS_ANYTHING_TO_BUILD=1
 BL31_MAKEARGS += BUILD_MESSAGE_TIMESTAMP='"$(shell sed -n 's/^.define COREBOOT_BUILD\>.*"\(.*\)".*/\1/p' $(obj)/build.h)"'
 
 BL31_CFLAGS := -fno-pic -fno-stack-protector -Wno-deprecated-declarations -Wno-unused-function
-BL31_LDFLAGS := --emit-relocs
+BL31_LDFLAGS := -Wl,--emit-relocs
 
 BL31 := $(obj)/bl31.elf
 
 $(BL31): $(obj)/build.h
 	printf "    MAKE       $(subst $(obj)/,,$(@))\n"
-	+unset AS AR CC CPP OC OD; \
-	LD=$(word 1, $(LD_arm64)) \
+	+unset AS AR CC CPP OC OD LD; \
 	CROSS_COMPILE="$(CROSS_COMPILE_arm64)" \
 	CFLAGS="$(BL31_CFLAGS)" \
 	LDFLAGS="$(BL31_LDFLAGS)" \