Add WITH_/WITHOUT_LLD knobs to enable the lld linker

[emaste]

Summary

Use this to control inclusion of the libllvm functionality required by lld. Enable by default on arm64 and amd64, the two platforms where lld is most usable for testing.

(Some additional functionality may have been added beyond the minimal required set. E.g. I added all of the LTO libs while it's possible only a subset is really needed. Probably not worth worrying about.)

[emaste]

lib/clang/libllvm/Makefile

1117–1119 ↗

(On Diff #19856)

Maybe this complexity isn't warranted and we should just add the lld requirements to SRCS_MIN?

[emaste]

And also this change:

diff --git a/usr.bin/clang/Makefile b/usr.bin/clang/Makefile
index d560a8a..c6ba2b3 100644
--- a/usr.bin/clang/Makefile
+++ b/usr.bin/clang/Makefile
@@ -33,6 +33,9 @@ SUBDIR+=      llvm-symbolizer
 SUBDIR+=       opt
 .endif
 
+.if ${MK_LLD} != "no"
+SUBDIR+=       lld
+.endif
 .if ${MK_LLDB} != "no"
 SUBDIR+=       lldb
 .endif # MK_LLDB

[emaste]

share/mk/src.opts.mk

251–253 ↗

(On Diff #19856)

For now it seems sensible to enable on amd64 (where lld is close to working) and arm64 (where we have no other linker in the base system), and perhaps add i386 and arm(32) when we bring in 4.0.

Perhaps we want to commit first with LLD as a default NO option and then follow up with a commit to enable on amd64 and arm64 to aid in the MFC process (i.e., plan to merge Clang/LLVM* 3.9 to stable/11 without turning LLD on by default).