Fix libspl/asm-generic/atomic for VPATH build

Fix libspl/asm-generic/atomic for VPATH build

Currently, asm-generic/atomic.c is compiled into a .S file, with a
comment saying this is to simplify the upper-level Makefile.

However, this doesn't work properly with a VPATH build, which would
require better logic to deal with generated sources correctly.

It also doesn't seem more complex to just specify the .c/.S source file,
depending on the cpu, instead of only the source directory in
lib/libspl/Makefile.am, which eliminates the need to do the intermediate
compilation.

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Closes #10493