[libc] Fix passing the full runtime target for multilibs

Summary:
The multilib builds pass `+name` to the runtime target. We were using
this as a strict triple and passing it to the compiler. Parse out the
triple portion in this case.

Author: jhuber6

libc/cmake/modules/LLVMLibCArchitectures.cmake

@@ -211,6 +211,12 @@ if(explicit_target_triple AND
             "GCC target triple (${libc_compiler_triple}) and the explicity "
             "specified target triple (${explicit_target_triple}) do not match.")
   else()
+    # Make sure we don't include any multilib specifiers.
+    string(FIND "${explicit_target_triple}" "+" plus_pos)
+    if(NOT plus_pos EQUAL -1)
+      string(SUBSTRING "${explicit_target_triple}" 0 ${plus_pos} explicit_target_triple)
+    endif()
+
     list(APPEND
          LIBC_COMPILE_OPTIONS_DEFAULT "--target=${explicit_target_triple}")
   endif()