rust-llvm: combine do_install_append()

There were two different do_install_append()'s and they didn't check for
errors so this improves that situation.

recipes-devtools/rust/rust-llvm.bb

@@ -1,9 +1,3 @@
 require rust-llvm.inc
 
 LIC_FILES_CHKSUM = "file://LICENSE.TXT;md5=4c0bc17c954e99fd547528d938832bfa"
-
-do_install_append () {
-	cd "${B}"
-	install -d "${D}${bindir}"
-	install -m755 "Release/bin/FileCheck" "${D}${bindir}"
-}