Define _LIBCPP_HAS_MUSL_LIBC in libcxx build (#17022)

This allows us to remove `include/compat/xlocale.h` which it seems was
added to support libcxx, but libcxx already has support for musl
internally when this macro is defined.

Author: sbc100

system/include/compat/xlocale.h

@@ -2,3 +2,4 @@
 // data and other nice fixes.
 #define _LIBCPP_ABI_VERSION 2
 #define _LIBCPP_HAS_NO_VENDOR_AVAILABILITY_ANNOTATIONS
+#define _LIBCPP_HAS_MUSL_LIBC

system/lib/libc/compat/strtol_l.c

@@ -34,8 +34,7 @@
 # include <__support/newlib/xlocale.h>
 #elif defined(__OpenBSD__)
 # include <__support/openbsd/xlocale.h>
-#elif (defined(__APPLE__)      || defined(__FreeBSD__) \
-    || defined(__EMSCRIPTEN__) || defined(__IBMCPP__))
+#elif (defined(__APPLE__) || defined(__FreeBSD__) || defined(__IBMCPP__))
 # include <xlocale.h>
 #elif defined(__Fuchsia__)
 # include <__support/fuchsia/xlocale.h>

system/lib/libcxx/include/__config_site

@@ -29,12 +29,12 @@
 // No _LIBCPP_ELAST needed on Fuchsia
 #elif defined(__wasi__)
 // No _LIBCPP_ELAST needed on WASI
+#elif defined(__EMSCRIPTEN__)
+// No _LIBCPP_ELAST needed on Emscripten
 #elif defined(__linux__) || defined(_LIBCPP_HAS_MUSL_LIBC)
 #define _LIBCPP_ELAST 4095
 #elif defined(__APPLE__)
 // No _LIBCPP_ELAST needed on Apple
-#elif defined(__EMSCRIPTEN__)
-// No _LIBCPP_ELAST needed on Emscripten
 #elif defined(__sun__)
 #define _LIBCPP_ELAST ESTALE
 #elif defined(__MVS__)