arch/arm64: Use -mno-implicit-float with clang

This fixes building inline assembly that uses SIMD registers. This is
for instance the case in the vboot library.

Change-Id: I24fa9d9f81430ea3ecd40de4304a10c6e235fece
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/81658
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>

toolchain.mk

@@ -60,7 +60,7 @@ ARCHDIR-riscv	:= riscv
 ARCHDIR-ppc64	:= ppc64
 
 CFLAGS_arm	+=
-CFLAGS_arm64	+= -mgeneral-regs-only
+CFLAGS_arm64	+=
 CFLAGS_riscv	+=
 CFLAGS_x86_32	+=
 CFLAGS_x86_64	+= -mcmodel=large -mno-red-zone

util/xcompile/xcompile

@@ -256,6 +256,11 @@ detect_special_flags() {
 		testld "$GCC" "$FLAGS_GCC" "${GCCPREFIX}ld${LINKER_SUFFIX}" \
 		  "$LDFLAGS --fix-cortex-a53-843419" && \
 		  LDFLAGS_ARM64_A53_ERRATUM_843419+=" --fix-cortex-a53-843419"
+		  # https://bugs.llvm.org/show_bug.cgi?id=30792
+		  # LLVM/clang runs into problems when encountering SIMD registers in assembly
+		  # with -mgeneral-regs-only. -mno-implicit-float achieves the same though.
+		  CFLAGS_GCC="$CFLAGS_GCC -mgeneral-regs-only" &&
+		  CFLAGS_CLANG="$CFLAGS_CLANG -mno-implicit-float"
 		;;
 	riscv)
 		testcc "$GCC" "$FLAGS_GCC -march=rv64iadc_zicsr_zifencei" &&