Use get_build_target consistently (#2861)

There were still a few places in the bootstrap script that were using a
hardcoded target triple.

rdar://problem/63036398

(cherry picked from commit ab03d78)

Author: neonichu

Utilities/bootstrap

@@ -430,7 +430,7 @@ def build_tsc(args):
 
     cmake_flags = []
     if platform.system() == 'Darwin':
-        cmake_flags.append("-DCMAKE_C_FLAGS=-target x86_64-apple-macosx%s" % g_macos_deployment_target)
+        cmake_flags.append("-DCMAKE_C_FLAGS=-target %s%s" % (get_build_target(args), g_macos_deployment_target))
         cmake_flags.append("-DCMAKE_OSX_DEPLOYMENT_TARGET=%s" % g_macos_deployment_target)
 
     build_with_cmake(args, cmake_flags, args.tsc_source_dir, args.tsc_build_dir)
@@ -445,7 +445,7 @@ def build_swiftpm_with_cmake(args):
     ]
 
     if platform.system() == 'Darwin':
-        cmake_flags.append("-DCMAKE_C_FLAGS=-target x86_64-apple-macosx%s" % g_macos_deployment_target)
+        cmake_flags.append("-DCMAKE_C_FLAGS=-target %s%s" % (get_build_target(args), g_macos_deployment_target))
         cmake_flags.append("-DCMAKE_OSX_DEPLOYMENT_TARGET=%s" % g_macos_deployment_target)
 
     build_with_cmake(args, cmake_flags, args.project_root, args.bootstrap_dir)