package/rustc: prepare for musl-based platforms support

The current rustc package only supports configurations based on glibc
and hardcodes this requirement. This patch prepares the addition of
support for musl-based platforms by using $(LIBC) instead of
hardcoding "gnu" as the C library specifier when defining
RUSTC_TARGET_NAME.

Signed-off-by: Nathaniel Husted <nathaniel.husted@outlook.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

package/rustc/rustc.mk

@@ -8,7 +8,7 @@ RUSTC_ARCH = $(call qstrip,$(BR2_PACKAGE_HOST_RUSTC_ARCH))
 RUSTC_ABI = $(call qstrip,$(BR2_PACKAGE_HOST_RUSTC_ABI))
 
 ifeq ($(BR2_PACKAGE_HOST_RUSTC_TARGET_ARCH_SUPPORTS),y)
-RUSTC_TARGET_NAME = $(RUSTC_ARCH)-unknown-linux-gnu$(RUSTC_ABI)
+RUSTC_TARGET_NAME = $(RUSTC_ARCH)-unknown-linux-$(LIBC)$(RUSTC_ABI)
 endif
 
 ifeq ($(HOSTARCH),x86)