Fix build on Solaris

Solaris builds were failing during async compilation because the .o files
created from compiling the corresponding .c files held in async/arch were
ending up in the top level async directory. Consequently the link fails
because it can't find the .o files.

Thanks to Richard Levitte for pointing me in the right direction on this.

Reviewed-by: Richard Levitte <levitte@openssl.org>