Add support for selectively enabling LLVM targets

This makes it possible, through src.conf(5) settings, to select which
LLVM targets you want to build during buildworld.  The current list is:

* (WITH|WITHOUT)_LLVM_TARGET_AARCH64
* (WITH|WITHOUT)_LLVM_TARGET_ARM
* (WITH|WITHOUT)_LLVM_TARGET_MIPS
* (WITH|WITHOUT)_LLVM_TARGET_POWERPC
* (WITH|WITHOUT)_LLVM_TARGET_SPARC
* (WITH|WITHOUT)_LLVM_TARGET_X86

To not influence anything right now, all of these are on by default, in
situations where clang is enabled.

Selectively turning a few targets off manually should work.  Turning on
only one target should work too, even if that target does not correspond
to the build architecture.  (In that case, LLVM_NATIVE_ARCH will not be
defined, and you can only use the resulting clang executable for
cross-compiling.)

I performed a few measurements on one of the FreeBSD.org reference
machines, building clang from scratch, with all targets enabled, and
with only the x86 target enabled.  The latter was ~12% faster in real
time (on a 32-core box), and ~14% faster in user time.  For a full
buildworld the difference will probably be less pronounced, though.

Reviewed by:	bdrewery
MFC after:	1 week
Differential Revision: https://reviews.freebsd.org/D11077

1 files changed, 2 insertions, 0 deletions

diff --git a/tools/build/options/WITH_LLVM_TARGET_ARM b/tools/build/options/WITH_LLVM_TARGET_ARM
new file mode 100644
index 000000000000..b57c6711f75e
--- /dev/null
+++ b/tools/build/options/WITH_LLVM_TARGET_ARM
@@ -0,0 +1,2 @@
+.\" $FreeBSD$
+Set to build LLVM target support for ARM.