Reenable build of compiler-rt with LLVM_ENABLE_RUNTIMES in Linux bots...

...we disabled in #81354

This requires a couple of supporting changes

* under Linux, do not cross compile LLVM when building for the host
 architecture -- that will ensure that the compiler-rt build will use
the just built compiler and not the system one (which may not be
new enough for this purpose);
* provide sanitizer flags depending on the linker the just built compiler
 will use -- this detection is brittle, so print a message advising the
user how to override this.

Addresses rdar://150849329

utils/swift_build_support/swift_build_support/products/llvm.py

@@ -242,7 +242,9 @@ class LLVM(cmake_product.CMakeProduct):
         llvm_cmake_options.define('INTERNAL_INSTALL_PREFIX', 'local')
 
         if host_target.startswith('linux'):
-            toolchain_file = self.generate_linux_toolchain_file(platform, arch)
+            toolchain_file = self.generate_linux_toolchain_file(
+                platform, arch,
+                crosscompiling=self.is_cross_compile_target(host_target))
             llvm_cmake_options.define('CMAKE_TOOLCHAIN_FILE:PATH', toolchain_file)
             if not self.is_release():
                 # On Linux build LLVM and subprojects with -gsplit-dwarf which is more
@@ -309,12 +311,26 @@ class LLVM(cmake_product.CMakeProduct):
         if host_target.startswith('linux'):
             # This preserves the behaviour we had when using
             # LLVM_BUILD_EXTERNAL COMPILER_RT --
-            # that is, having the linker not complaing if symbols used
+            # that is, having the linker not complaining if symbols used
             # by TSan are undefined (namely the ones for Blocks Runtime)
             # In the long term, we want to remove this and
             # build Blocks Runtime before LLVM
-            llvm_cmake_options.define(
+            if ("-DCLANG_DEFAULT_LINKER=gold" in llvm_cmake_options
-                'SANITIZER_COMMON_LINK_FLAGS:STRING', '-Wl,-z,undefs')
+                or "-DCLANG_DEFAULT_LINKER:STRING=gold" in llvm_cmake_options):
+                print("Assuming just built clang will use a gold linker -- "
+                      "if that's not the case, please adjust the value of "
+                      "`SANITIZER_COMMON_LINK_FLAGS` in `extra-llvm-cmake-options`",
+                      flush=True)
+                llvm_cmake_options.define(
+                    'SANITIZER_COMMON_LINK_FLAGS:STRING',
+                    '-Wl,--unresolved-symbols,ignore-in-object-files')
+            else:
+                print("Assuming just built clang will use a non gold linker -- "
+                      "if that's not the case, please adjust the value of "
+                      "`SANITIZER_COMMON_LINK_FLAGS` in `extra-llvm-cmake-options`",
+                      flush=True)
+                llvm_cmake_options.define(
+                    'SANITIZER_COMMON_LINK_FLAGS:STRING', '-Wl,-z,undefs')
 
         builtins_runtimes_target_for_darwin = f'{arch}-apple-darwin'
         if system() == "Darwin":

utils/swift_build_support/swift_build_support/products/product.py

@@ -389,7 +389,7 @@ class Product(object):
         sysroot_arch, vendor, abi = self.get_linux_target_components(arch)
         return '{}-{}-linux-{}'.format(sysroot_arch, vendor, abi)
 
-    def generate_linux_toolchain_file(self, platform, arch):
+    def generate_linux_toolchain_file(self, platform, arch, crosscompiling=True):
         """
         Generates a new CMake tolchain file that specifies Linux as a target
         platform.
@@ -402,8 +402,9 @@ class Product(object):
 
         toolchain_args = {}
 
-        toolchain_args['CMAKE_SYSTEM_NAME'] = 'Linux'
+        if crosscompiling:
-        toolchain_args['CMAKE_SYSTEM_PROCESSOR'] = arch
+            toolchain_args['CMAKE_SYSTEM_NAME'] = 'Linux'
+            toolchain_args['CMAKE_SYSTEM_PROCESSOR'] = arch
 
         # We only set the actual sysroot if we are actually cross
         # compiling. This is important since otherwise cmake seems to change the