build: Suffix build directory with _$(LIBC) for external toolchains

For external toolchain, we also know the type of C library used, and the
toolchain triplet may not always be reflective of that, therefore make
$(TARGET_DIR_NAME) suffixed with _$(LIBC).

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>

rules.mk

@@ -134,7 +134,8 @@ else
     GNU_TARGET_NAME=$(shell gcc -dumpmachine)
   endif
   REAL_GNU_TARGET_NAME=$(GNU_TARGET_NAME)
-  TARGET_DIR_NAME:=target-$(GNU_TARGET_NAME)$(if $(BUILD_SUFFIX),_$(BUILD_SUFFIX))
+  LIBC:=$(call qstrip,$(CONFIG_LIBC))
+  TARGET_DIR_NAME:=target-$(GNU_TARGET_NAME)_$(LIBC)$(if $(BUILD_SUFFIX),_$(BUILD_SUFFIX))
   TOOLCHAIN_DIR_NAME:=toolchain-$(GNU_TARGET_NAME)
 endif