[libcxx] Define _LIBCPP_HAS_C8RTOMB_MBRTOC8 for picolibc

Starting from picolibc 1.8.9, the `char8_t` related functions are
provided.

This patch adds logic to detect the underlying picolibc version and
define the `_LIBCPP_HAS_C8RTOMB_MBRTOC8 macro` accordingly.

Author: vhscampos

libcxx/include/__config

@@ -1027,8 +1027,12 @@ typedef __char32_t char32_t;
 #    else
 #      define _LIBCPP_HAS_C8RTOMB_MBRTOC8 0
 #    endif
-#  else
-#    define _LIBCPP_HAS_C8RTOMB_MBRTOC8 0
+#  elif defined (_LIBCPP_PICOLIBC_PREREQ)
+#    if _LIBCPP_PICOLIBC_PREREQ(1, 8, 9) && defined(__cpp_char8_t)
+#      define _LIBCPP_HAS_C8RTOMB_MBRTOC8 1
+#    else
+#      define _LIBCPP_HAS_C8RTOMB_MBRTOC8 0
+#    endif
 #  endif
 
 // There are a handful of public standard library types that are intended to

libcxx/include/__configuration/platform.h

@@ -47,6 +47,10 @@
 //       user code. Move code paths that need _NEWLIB_VERSION to another customization mechanism.
 #if __has_include(<picolibc.h>)
 #  include <picolibc.h>
+#  define _LIBCPP_PICOLIBC_VERSION_INT(maj, min, patch) (maj * 10000 + min * 100 + patch)
+#  define _LIBCPP_PICOLIBC_PREREQ(maj, min, patch)                                                                     \
+    _LIBCPP_PICOLIBC_VERSION_INT(__PICOLIBC__, __PICOLIBC_MINOR__, __PICOLIBC_PATCHLEVEL__) >=                         \
+        _LIBCPP_PICOLIBC_VERSION_INT(maj, min, patch)
 #endif
 
 #ifndef __BYTE_ORDER__