libc/picolibc: Clean up Picolibc Kconfig for C++

libstdc++ is supported with Picolibc only when the toolchain version of
Picolibc is use -- libstdc++ must be built using a specific Picolibc build
and libstdc++ is included with the toolchain.

Ideally, we'd allow the use of the Picolibc module whenever we weren't
using the GNU libstdc++, including when using the minimal libc++. However,
the obvious dependency settings create a loop:

config PICOLIBC
    depends on PICOLIBC_SUPPORTED

config PICOLIBC_SUPPORTED
    depends on !(GLIBCXX_LIBCPP && "$(ZEPHYR_TOOCHAIN_VARIANT" = "zephyr")

config GLIBCXX_LIBCPP
    depends on NEWLIB_LIBC || PICOLIBC

To break this loop, we replace GLIBCXX_LIBCPP in the second block with
CPP:

config PICOLIBC_SUPPORTED
    depends on !(CPP && "$(ZEPHYR_TOOCHAIN_VARIANT" = "zephyr")

This means that picolibc cannot be used with any C++ apps when using the
Zephyr SDK, even when not using the GNU libstdc++. However, Zephyr SDK 0.16
will come with an additional Kconfig file that includes:

config PICOLIBC_SUPPORTED
    def_bool y
    depends on "$(ZEPHYR_TOOLCHAIN_VARIANT)" = "zephyr"

This will override the Kconfig bits included in Zephyr and allow use of the
Picolibc module with C++ code, including using the minimal libc++ bits.

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

lib/cpp/Kconfig

@@ -85,7 +85,7 @@ config MINIMAL_LIBCPP
 config GLIBCXX_LIBCPP
 	bool "GNU C++ Standard Library"
 	depends on !NATIVE_APPLICATION
-	depends on NEWLIB_LIBC || (PICOLIBC && !PICOLIBC_USE_MODULE)
+	depends on NEWLIB_LIBC || PICOLIBC
 	help
 	  Build with GNU C++ Standard Library (libstdc++) provided by the GNU
 	  Compiler Collection (GCC)-based toolchain.

lib/libc/Kconfig

@@ -15,10 +15,12 @@ config SUPPORT_MINIMAL_LIBC
 	bool
 	default y
 
+# Picolibc with C++ support in Zephyr SDK is handled by Zephyr SDK's own Kconfig.
 config PICOLIBC_SUPPORTED
 	bool
 	depends on ARC || ARM || ARM64 || MIPS || RISCV
 	depends on "$(ZEPHYR_TOOLCHAIN_VARIANT)" != "arcmwdt"
+	depends on !(CPP && "$(ZEPHYR_TOOLCHAIN_VARIANT)" = "zephyr")
 	default y
 	help
 	  Selected when the target has support for picolibc.
@@ -42,6 +44,7 @@ config PICOLIBC
 	select THREAD_LOCAL_STORAGE if ARCH_HAS_THREAD_LOCAL_STORAGE && TOOLCHAIN_SUPPORTS_THREAD_LOCAL_STORAGE
 	select LIBC_ERRNO if THREAD_LOCAL_STORAGE
 	depends on !NATIVE_APPLICATION
+	depends on PICOLIBC_SUPPORTED
 	help
 	  Build with picolibc library. The picolibc library is built as
 	  a module if PICOLIBC_MODULE is set, otherwise picolibc is

lib/libc/picolibc/Kconfig

@@ -4,9 +4,10 @@
 if PICOLIBC
 
 config PICOLIBC_USE_MODULE
-	bool "Use picolibc module"
+	bool "Picolibc as module"
 	default y
-	select PICOLIBC_MODULE
+	depends on ZEPHYR_PICOLIBC_MODULE
+	depends on !GLIBCXX_LIBCPP
 	help
 	  Use picolibc module instead of picolibc included with toolchain