The LOCK_OBJ_DEF macro is used with a trailing semicolon. However,
since the macro definition ends with the closing brace of a function
definition, the ISO C grammar does not allow an extra semicolon.
To fix this, swap the order of the two definitions, and drop the
semicolon from the __malloc_lock declaration.
This enables alternative compilers, which may not define __GNUC__,
to implement alloca, which is still fairly widely used.
This is similar to how stdarg.h already works in musl; compilers must
implement __builtin_va_arg, there is no fallback definition.
