Change Linux/ARM triples. Previous values were not valid (#5460)

Resolves https://bugs.swift.org/browse/SR-3019.

Author: karwa

utils/build-script-impl

@@ -432,12 +432,12 @@ function set_build_options_for_host() {
                     ;;
                 linux-armv6)
                     SWIFT_HOST_VARIANT_ARCH="armv6"
-                    SWIFT_HOST_TRIPLE="armv6-linux-gnueabihf"
+                    SWIFT_HOST_TRIPLE="armv6-unknown-linux-gnueabihf"
                     llvm_target_arch="ARM"
                     ;;
                 linux-armv7)
                     SWIFT_HOST_VARIANT_ARCH="armv7"
-                    SWIFT_HOST_TRIPLE="armv7-linux-gnueabihf"
+                    SWIFT_HOST_TRIPLE="armv7-unknown-linux-gnueabihf"
                     llvm_target_arch="ARM"
                     ;;
                 linux-aarch64)