libc/picolibc: Allow toolchain picolibc use with C++

The Zephyr SDK includes C++ support for picolibc when using picolibc
packaged with the SDK.

Signed-off-by: Keith Packard <keithp@keithp.com>

Author: keith-packard

lib/libc/Kconfig

@@ -19,6 +19,7 @@ config PICOLIBC_SUPPORTED
 	bool
 	depends on ARC || ARM || ARM64 || MIPS || RISCV
 	depends on "$(ZEPHYR_TOOLCHAIN_VARIANT)" != "arcmwdt"
+	depends on !CPLUSPLUS || !PICOLIBC_MODULE
 	default y
 	help
 	  Selected when the target has support for picolibc.