Make: newlib: use sh instead of exec/fork

sys/newlib/Makefile.include

@@ -48,7 +48,7 @@ NEWLIB_INCLUDE_DIR ?= $(firstword $(wildcard $(NEWLIB_INCLUDE_PATTERNS)))
 # If nothing was found we will try to fall back to searching for a cross-gcc in
 # the current PATH and use a relative path for the includes
 ifeq (,$(NEWLIB_INCLUDE_DIR))
-  NEWLIB_INCLUDE_DIR := $(abspath $(wildcard $(dir $(shell command -v $(PREFIX)gcc))../$(TARGET_ARCH)/include))
+  NEWLIB_INCLUDE_DIR := $(abspath $(wildcard $(dir $(shell command -v $(PREFIX)gcc;))../$(TARGET_ARCH)/include))
 endif
 
 ifeq ($(TOOLCHAIN),llvm)