RISC-V: Add multiarch support on riscv-linux-gnu

This adds multiarch support to the RISC-V port so that bootstraps work with
Debian out-of-the-box.  Without this patch the stage1 compiler is unable to
find headers/libraries when building the stage1 runtime.

This is functionally (and possibly textually) equivalent to Debian's fix for
the same problem.

gcc/
	* config/riscv/t-linux: Add MULTIARCH_DIRNAME.

Author: rzinsly

gcc/config/riscv/t-linux

@@ -1,3 +1,5 @@
 # Only XLEN and ABI affect Linux multilib dir names, e.g. /lib32/ilp32d/
 MULTILIB_DIRNAMES := $(patsubst rv32%,lib32,$(patsubst rv64%,lib64,$(MULTILIB_DIRNAMES)))
 MULTILIB_OSDIRNAMES := $(patsubst lib%,../lib%,$(MULTILIB_DIRNAMES))
+
+MULTIARCH_DIRNAME := $(call if_multiarch,$(firstword $(subst -, ,$(target)))-linux-gnu)