scripts/Makefile.clang: default to LLVM_IAS=1

LLVM_IAS=1 controls enabling clang's integrated assembler via
-integrated-as. This was an explicit opt in until we could enable
assembler support in Clang for more architecures. Now we have support
and CI coverage of LLVM_IAS=1 for all architecures except a few more
bugs affecting s390 and powerpc.

This commit flips the default from opt in via LLVM_IAS=1 to opt out via
LLVM_IAS=0. CI systems or developers that were previously doing builds
with CC=clang or LLVM=1 without explicitly setting LLVM_IAS must now
explicitly opt out via LLVM_IAS=0, otherwise they will be implicitly
opted-in.

This finally shortens the command line invocation when cross compiling
with LLVM to simply:

$ make ARCH=arm64 LLVM=1

Signed-off-by: Nick Desaulniers <[e-mail]>
Reviewed-by: Nathan Chancellor <[e-mail]>
Signed-off-by: Masahiro Yamada <[e-mail]>

Documentation/kbuild/llvm.rst

···
 	  OBJCOPY=llvm-objcopy OBJDUMP=llvm-objdump READELF=llvm-readelf \
 	  HOSTCC=clang HOSTCXX=clang++ HOSTAR=llvm-ar HOSTLD=ld.lld
 
-Currently, the integrated assembler is disabled by default. You can pass
-``LLVM_IAS=1`` to enable it.
+The integrated assembler is enabled by default. You can pass ``LLVM_IAS=0`` to
+disable it.
 
 Omitting CROSS_COMPILE
 ----------------------
 
 As explained above, ``CROSS_COMPILE`` is used to set ``--target=<triple>``.
-
-Unless ``LLVM_IAS=1`` is specified, ``CROSS_COMPILE`` is also used to derive
-``--prefix=<path>`` to search for the GNU assembler and linker.
 
 If ``CROSS_COMPILE`` is not specified, the ``--target=<triple>`` is inferred
 from ``ARCH``.
···
 
 For example, to cross-compile the arm64 kernel::
 
-	make ARCH=arm64 LLVM=1 LLVM_IAS=1
+	make ARCH=arm64 LLVM=1
+
+If ``LLVM_IAS=0`` is specified, ``CROSS_COMPILE`` is also used to derive
+``--prefix=<path>`` to search for the GNU assembler and linker. ::
+
+	make ARCH=arm64 LLVM=1 LLVM_IAS=0 CROSS_COMPILE=aarch64-linux-gnu-
 
 Supported Architectures
 -----------------------

scripts/Makefile.clang

···
 CLANG_FLAGS	+= --target=$(notdir $(CROSS_COMPILE:%-=%))
 endif # CROSS_COMPILE
 
-ifeq ($(LLVM_IAS),1)
-CLANG_FLAGS	+= -integrated-as
-else
+ifeq ($(LLVM_IAS),0)
 CLANG_FLAGS	+= -no-integrated-as
 GCC_TOOLCHAIN_DIR := $(dir $(shell which $(CROSS_COMPILE)elfedit))
 CLANG_FLAGS	+= --prefix=$(GCC_TOOLCHAIN_DIR)$(notdir $(CROSS_COMPILE))
+else
+CLANG_FLAGS	+= -integrated-as
 endif
 CLANG_FLAGS	+= -Werror=unknown-warning-option
 KBUILD_CFLAGS	+= $(CLANG_FLAGS)