LDC: Add compiler-rt as git submodule

Including minor fixes to build the 'builtins' library properly for
Windows with clang-cl. Building with clang is required to make the
library include some objects implemented in ASM, as the MS assembler
doesn't support the used AT&T syntax.

There were some more minor annoyances standing in the way of using LDC
with MinGW-w64-based libs.

Author: kinke

.gitignore

@@ -50,7 +50,7 @@ autoconf/autom4te.cache
 # Directories to ignore (do not add trailing '/'s, they skip symlinks).
 #==============================================================================#
 # External projects that are tracked independently.
-projects/*
+#projects/*
 !projects/*.*
 !projects/Makefile
 runtimes/*

.gitmodules

@@ -1,3 +1,6 @@
 [submodule "lld"]
 	path = tools/lld
 	url = https://github.com/ldc-developers/lld.git
+[submodule "projects/compiler-rt"]
+	path = projects/compiler-rt
+	url = https://github.com/ldc-developers/compiler-rt.git