Linux ppc64le ieee128 compat: Do not redefine __asm on external headers

There is an external assembly declaration extension in GNU C that glibc
uses when building with ieee128 floating point support on ppc64le.
Marking that as volatile makes no sense, so the build breaks.

It does not make sense to only mark this as volatile on Linux, since if
do not want the compiler reordering things on Linux, we do not want the
compiler reordering things on any other platform, so we stop treating
Linux specially and just manually inline the CPP macro so that we can
eliminate it. This should fix the build on ppc64le.

Tested-by: @gyakovlev
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #14308
Closes #14384