[PATCH] arm64: drop CROSS_COMPILE for LLVM=1 LLVM_IAS=1

[Nick Desaulniers]

We get constant feedback that the command line invocation of make is too
long. CROSS_COMPILE is helpful when a toolchain has a prefix of the
target triple, or is an absolute path outside of $PATH, but it's mostly
redundant for a given ARCH.

If CROSS_COMPILE is not set, simply set --target=aarch64-linux for
CLANG_FLAGS, KBUILD_CFLAGS, and KBUILD_AFLAGS.

Previously, we'd cross compile via:
$ ARCH=arm64 CROSS_COMPILE=aarch64-linxu-gnu make LLVM=1 LLVM_IAS=1
Now:
$ ARCH=arm64 make LLVM=1 LLVM_IAS=1

We can drop gnu from the triple, but dropping linux from the triple
produces different .config files for the above invocations for the
defconfig target.

Link: https://github.com/ClangBuiltLinux/linux/issues/1399
Suggested-by: Arnd Bergmann <arnd@xxxxxxxxxx>
Suggested-by: Fangrui Song <maskray@xxxxxxxxxx>
Signed-off-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
---
 arch/arm64/Makefile | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile
index 7bc37d0a1b68..016873fddcc3 100644
--- a/arch/arm64/Makefile
+++ b/arch/arm64/Makefile
@@ -34,6 +34,17 @@ $(warning LSE atomics not supported by binutils)
   endif
 endif
 
+ifneq ($(LLVM),)
+ifneq ($(LLVM_IAS),)
+ifeq ($(CROSS_COMPILE),)
+CLANG_TARGET	:=--target=aarch64-linux
+CLANG_FLAGS	+= $(CLANG_TARGET)
+KBUILD_CFLAGS	+= $(CLANG_TARGET)
+KBUILD_AFLAGS	+= $(CLANG_TARGET)
+endif
+endif
+endif
+
 cc_has_k_constraint := $(call try-run,echo				\
 	'int main(void) {						\
 		asm volatile("and w0, w0, %w0" :: "K" (4294967295));	\
-- 
2.32.0.93.g670b81a890-goog