package/rustc: needs C++ compiler with musl based toolchain

While building host-rust with a musl based toolchain without C++ compiler,
the build fail since libunwind bundled in rust sources needs a C++ compiler.

cargo:warning=i686-buildroot-linux-musl-gcc.br_real: error: [...]/host-rust-1.62.0/src/llvm-project/libunwind/src/Unwind-EHABI.cpp: C++ compiler not installed on this system

Note: the issues can't be reproduced with a glibc based toolchain
without C++ probaly due to extra steps required to support musl libc.

We could add the C++ dependency direclty to host-rustc but it would
requires adding the C++ reverse dependencies to all rust packages.

Instread, we add the C++ dependency to BR2_PACKAGE_HOST_RUSTC_TARGET_ARCH_SUPPORTS
only when a musl toolchain is used. So we can still install a prebuilt
rust compiler but without the rust standard library (rust-std).

Usually we should not add toolchain dependencies in a _ARCH_SUPPORTS option but
BR2_PACKAGE_HOST_RUSTC_TARGET_TIER... options contains already some
BR2_TOOLCHAIN_USES_GLIBC or BR2_TOOLCHAIN_USES_MUSL.

Fixes:
http://autobuild.buildroot.org/results/636/636fb39c8f1b8c05e4ca451ac506cd63c7166d82

Signed-off-by: Romain Naour <romain.naour@smile.fr>
Reviewed-by: Nicolas Tran <nicolas.tran@smile.fr>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
(cherry picked from commit 8520058ac097284dd4c5676e264fc1aa444c1127)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/rustc/Config.in.host

@@ -110,11 +110,15 @@ config BR2_PACKAGE_HOST_RUSTC_TARGET_TIER2_PLATFORMS
 	default y if BR2_sparc64 && BR2_TOOLCHAIN_USES_GLIBC
 
 # All target rust packages should depend on this option
+# Note: With musl based toolchain, we need a C++ cross compiler to
+# build host-rust some target libraries (libunwind) from llvm source
+# code bundled in rust sources.
 config BR2_PACKAGE_HOST_RUSTC_TARGET_ARCH_SUPPORTS
 	bool
 	default y if BR2_PACKAGE_HOST_RUSTC_TARGET_TIER1_PLATFORMS
 	default y if BR2_PACKAGE_HOST_RUSTC_TARGET_TIER2_HOST_TOOLS_PLATFORMS
 	default y if BR2_PACKAGE_HOST_RUSTC_TARGET_TIER2_PLATFORMS
+	depends on BR2_INSTALL_LIBSTDCPP || !BR2_TOOLCHAIN_USES_MUSL
 	depends on BR2_PACKAGE_HOST_RUSTC_ARCH_SUPPORTS
 
 config BR2_PACKAGE_HOST_RUSTC_ARCH