[libcxx] Add WebAssembly support

It turns out that this is the only change required in libcxx
for it to compile with the new `wasm32-unknown-unknown-wasm`
target recently added to Clang.

Patch by Nicholas Wilson!

Differential Revision: https://reviews.llvm.org/D41073

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@320925 91177308-0d34-0410-b5e6-96231b3b80d8

include/__config

@@ -45,6 +45,8 @@
 #define _LIBCPP_OBJECT_FORMAT_MACHO 1
 #elif defined(_WIN32)
 #define _LIBCPP_OBJECT_FORMAT_COFF  1
+#elif defined(__wasm__)
+#define _LIBCPP_OBJECT_FORMAT_WASM  1
 #else
 #error Unknown object file format
 #endif

src/include/config_elast.h

@@ -24,7 +24,7 @@
 #define _LIBCPP_ELAST __ELASTERROR
 #elif defined(__Fuchsia__)
 // No _LIBCPP_ELAST needed on Fuchsia
-#elif defined(__linux__)
+#elif defined(__linux__) || defined(_LIBCPP_HAS_MUSL_LIBC)
 #define _LIBCPP_ELAST 4095
 #elif defined(__APPLE__)
 // No _LIBCPP_ELAST needed on Apple