Only bring over supporting change of generate_linux_toolchain_file() from fcc0389,

specifically the change that makes the following possible on linux:

    "* 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);"

Author: finagolfin

utils/swift_build_support/swift_build_support/products/product.py

@@ -389,7 +389,7 @@ def get_linux_target(self, platform, arch):
         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 @@ def generate_linux_toolchain_file(self, platform, arch):
 
         toolchain_args = {}
 
-        toolchain_args['CMAKE_SYSTEM_NAME'] = 'Linux'
-        toolchain_args['CMAKE_SYSTEM_PROCESSOR'] = arch
+        if crosscompiling:
+            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