Bug 1492663 - Upgrade most CI builds to clang 7 r=froydnj

The cctools-port linker links against libraries from clang (for LTO),
which have different SONAMEs depending on the clang version. Which means
the linker needs to be used along the same version of clang it was built
against. Thus we also make it depend on linux64-clang-7.

But changing the dependency is not enough, cf. bug 1471905, so also
touch its build script, which it turns out, we need to do anyways
because llvm-dsymutil was renamed to dsymutil.

Relatedly, all toolchains that are built using cctools-port need to use
linux64-clang-7 too.

Building compiler-rt 7 with the OSX 10.11 SDK fails because of some
newer APIs being used in compiler-rt for xray, but this is not a feature
we use, so disable that.

Differential Revision: https://phabricator.services.mozilla.com/D6766

build/build-clang/build-clang.py

@@ -235,6 +235,8 @@ def build_one_stage(cc, cxx, asm, ld, ar, ranlib, libtool,
                 "-DCMAKE_SYSTEM_NAME=Darwin",
                 "-DCMAKE_SYSTEM_VERSION=10.10",
                 "-DLLVM_ENABLE_THREADS=OFF",
+                # Xray requires a OSX 10.12 SDK (https://bugs.llvm.org/show_bug.cgi?id=38959)
+                "-DCOMPILER_RT_BUILD_XRAY=OFF",
                 "-DLIBCXXABI_LIBCXX_INCLUDES=%s" % libcxx_include_dir,
                 "-DCMAKE_OSX_SYSROOT=%s" % slashify_path(os.getenv("CROSS_SYSROOT")),
                 "-DCMAKE_FIND_ROOT_PATH=%s" % slashify_path(os.getenv("CROSS_CCTOOLS_PATH")), # noqa