ARC: Support building of multi-lib Glibc toolchain

From GCC's standpoint ARC's multilib items are defined by "mcpu" values
which we have quite a few and for all of them might be built optimized
cross-toolchain.

From Glibc's standpoint multilib is just multi-ABI [1] and so very limited
versions are supposed to co-exist (e.g. arc700 & archs).

Here we force Glibc to install libraries in GCC's multilib folder to create
a universal cross-toolchain that has libs optimized for multiple CPU types.

But note we only need to mess with installation paths in case of real
multilib, otherwise we keep default "lib/" paths so that GCC finds default
(the one and only) libs where it expects them to be.

Also here we add a sample which allows to build universal Glibc Linux
toolchain for ARC.

[1] https://sourceware.org/ml/libc-alpha/2019-06/msg00018.html

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>

samples/arc-multilib-linux-gnu/crosstool.config

@@ -0,0 +1,7 @@
+CT_CONFIG_VERSION="3"
+CT_ARCH_ARC=y
+CT_MULTILIB=y
+CT_TARGET_CFLAGS="-matomic"
+CT_TARGET_VENDOR="multilib"
+CT_TARGET_ALIAS="arc-linux"
+CT_KERNEL_LINUX=y

samples/arc-multilib-linux-gnu/reported.by

@@ -0,0 +1,3 @@
+reporter_name="Alexey Brodkin"
+reporter_url="http://embarc.org"
+reporter_comment="ARC multilib GNU Linux toolchain"

scripts/build/arch/arc.sh

@@ -29,3 +29,39 @@ CT_DoArchUClibcCflags()
         esac
     done
 }
+
+# Multilib: Adjust configure arguments for GLIBC
+# Usage: CT_DoArchGlibcAdjustConfigure <configure-args-array-name> <cflags>
+#
+# From GCC's standpoint ARC's multilib items are defined by "mcpu" values
+# which we have quite a few and for all of them might be built optimized
+# cross-toolchain.
+#
+# From Glibc's standpoint multilib is multi-ABI and so very limited
+# versions are supposed to co-exist.
+#
+# Here we force Glibc to install libraries in per-multilib folder to create
+# a universal cross-toolchain that has libs optimized for multiple CPU types.
+CT_DoArchGlibcAdjustConfigure() {
+    local -a add_args
+    local array="${1}"
+    local cflags="${2}"
+    local opt
+    local mcpu
+
+    # If building for multilib, set proper installation paths
+    if [ "${CT_MULTILIB}" = "y" ]; then
+        for opt in ${cflags}; do
+            case "${opt}" in
+            -mcpu=*)
+                mcpu="${opt#*=}"
+                add_args+=( "libc_cv_rtlddir=/lib/${mcpu}" )
+                add_args+=( "libc_cv_slibdir=/lib/${mcpu}" )
+                add_args+=( "--libdir=/usr/lib/${mcpu}" )
+                ;;
+            esac
+        done
+    fi
+
+    eval "${array}+=( \"\${add_args[@]}\" )"
+}