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>