Index: Makefile.inc1
===================================================================
--- Makefile.inc1
+++ Makefile.inc1
@@ -130,6 +130,15 @@
 TARGET_ABI?=	unknown
 TARGET_TRIPLE?=	${TARGET_ARCH:S/amd64/x86_64/:C/hf$//:S/mipsn32/mips64/}-${TARGET_ABI}-freebsd12.0
 
+# If all targets are disabled for system llvm then don't expect it to work
+# for cross-builds.
+.if ${MK_LLVM_TARGET_ALL} == "no" && \
+    ${MACHINE} != ${TARGET} && ${MACHINE_ARCH} != ${TARGET_ARCH} && \
+    !make(showconfig)
+MK_SYSTEM_COMPILER=	no
+MK_SYSTEM_LINKER=	no
+.endif
+
 # Handle external binutils.
 .if defined(CROSS_TOOLCHAIN_PREFIX)
 CROSS_BINUTILS_PREFIX?=${CROSS_TOOLCHAIN_PREFIX}
Index: share/mk/src.opts.mk
===================================================================
--- share/mk/src.opts.mk
+++ share/mk/src.opts.mk
@@ -211,6 +211,7 @@
 # RIGHT option is disabled.
 __DEFAULT_DEPENDENT_OPTIONS= \
 	CLANG_FULL/CLANG \
+	LLVM_TARGET_ALL/CLANG \
 
 # MK_*_SUPPORT options which default to "yes" unless their corresponding
 # MK_* variable is set to "no".
@@ -249,6 +250,27 @@
 __TT=${MACHINE}
 .endif
 
+# All supported backends for LLVM_TARGET_XXX
+__LLVM_TARGETS= \
+		aarch64 \
+		arm \
+		mips \
+		powerpc \
+		sparc \
+		x86
+__LLVM_TARGET_FILT=	C/(amd64|i386)/x86/:S/sparc64/sparc/:S/arm64/aach64/
+# Default the given TARGET_ARCH's LLVM_TARGET support to the value of
+# MK_CLANG.
+# Default the rest of the LLVM_TARGETs to the value of MK_LLVM_TARGET_ALL
+# which is based on MK_CLANG.
+.for __llt in ${__LLVM_TARGETS}
+.if ${__llt} != ${__T:${__LLVM_TARGET_FILT}}
+__DEFAULT_DEPENDENT_OPTIONS+=	LLVM_TARGET_${__llt:${__LLVM_TARGET_FILT}:tu}/LLVM_TARGET_ALL
+.else
+__DEFAULT_DEPENDENT_OPTIONS+=	LLVM_TARGET_${__llt:${__LLVM_TARGET_FILT}:tu}/CLANG
+.endif
+.endfor
+
 .include <bsd.compiler.mk>
 # If the compiler is not C++11 capable, disable Clang and use GCC instead.
 # This means that architectures that have GCC 4.2 as default can not
@@ -258,23 +280,17 @@
     ${__T} == "amd64" || ${__TT} == "arm" || ${__T} == "i386")
 # Clang is enabled, and will be installed as the default /usr/bin/cc.
 __DEFAULT_YES_OPTIONS+=CLANG CLANG_BOOTSTRAP CLANG_IS_CC LLD
-__DEFAULT_YES_OPTIONS+=LLVM_TARGET_AARCH64 LLVM_TARGET_ARM LLVM_TARGET_MIPS
-__DEFAULT_YES_OPTIONS+=LLVM_TARGET_POWERPC LLVM_TARGET_SPARC LLVM_TARGET_X86
 __DEFAULT_NO_OPTIONS+=GCC GCC_BOOTSTRAP GNUCXX GPL_DTC
 .elif ${COMPILER_FEATURES:Mc++11} && ${__T:Mriscv*} == "" && ${__T} != "sparc64"
 # If an external compiler that supports C++11 is used as ${CC} and Clang
 # supports the target, then Clang is enabled but GCC is installed as the
 # default /usr/bin/cc.
 __DEFAULT_YES_OPTIONS+=CLANG GCC GCC_BOOTSTRAP GNUCXX GPL_DTC LLD
-__DEFAULT_YES_OPTIONS+=LLVM_TARGET_AARCH64 LLVM_TARGET_ARM LLVM_TARGET_MIPS
-__DEFAULT_YES_OPTIONS+=LLVM_TARGET_POWERPC LLVM_TARGET_SPARC LLVM_TARGET_X86
 __DEFAULT_NO_OPTIONS+=CLANG_BOOTSTRAP CLANG_IS_CC
 .else
 # Everything else disables Clang, and uses GCC instead.
 __DEFAULT_YES_OPTIONS+=GCC GCC_BOOTSTRAP GNUCXX GPL_DTC
 __DEFAULT_NO_OPTIONS+=CLANG CLANG_BOOTSTRAP CLANG_IS_CC LLD
-__DEFAULT_NO_OPTIONS+=LLVM_TARGET_AARCH64 LLVM_TARGET_ARM LLVM_TARGET_MIPS
-__DEFAULT_NO_OPTIONS+=LLVM_TARGET_POWERPC LLVM_TARGET_SPARC LLVM_TARGET_X86
 .endif
 # In-tree binutils/gcc are older versions without modern architecture support.
 .if ${__T} == "aarch64" || ${__T:Mriscv*} != ""
Index: tools/build/options/WITHOUT_LLVM_TARGET_ALL
===================================================================
--- /dev/null
+++ tools/build/options/WITHOUT_LLVM_TARGET_ALL
@@ -0,0 +1,3 @@
+.\" $FreeBSD$
+Set to only build the required LLVM target support.
+This option is preferred to specific target support options.
Index: tools/build/options/WITH_LLVM_TARGET_ALL
===================================================================
--- /dev/null
+++ tools/build/options/WITH_LLVM_TARGET_ALL
@@ -0,0 +1,2 @@
+.\" $FreeBSD$
+Set to build support for all LLVM targets.