Fix aarch64 toolchain detection and add missing compiler_builtins v0.1.5

kernel/Makefile

@@ -141,7 +141,9 @@ else ifeq ($(arch), riscv64)
 prefix := riscv64-unknown-elf-
 else ifeq ($(arch), aarch64)
 prefix ?= aarch64-none-elf-
-hash $(prefix)ld 2>/dev/null || {prefix := aarch64-elf- }
+ifeq (,$(shell which $(prefix)ld))
+	prefix := aarch64-elf-
+endif
 endif
 
 ld := $(prefix)ld

kernel/build-rv64

@@ -102,6 +102,9 @@ then
 	elif [[ -d $CARGO_PATH/registry/src/github.com-1ecc6299db9ec823/compiler_builtins-0.1.4 ]]
 	then
 		COMPILER_BUILTINS_PATH=$CARGO_PATH/registry/src/github.com-1ecc6299db9ec823/compiler_builtins-0.1.4
+	elif [[ -d $CARGO_PATH/registry/src/github.com-1ecc6299db9ec823/compiler_builtins-0.1.5 ]]
+	then
+		COMPILER_BUILTINS_PATH=$CARGO_PATH/registry/src/github.com-1ecc6299db9ec823/compiler_builtins-0.1.5
 	else
 		echo "Cannot find compiler_builtins crate! Please file an issue report"
 	fi