gptzfsboot: remove CLANG_NO_IAS

Many components under stand/ had CLANG_NO_IAS added when Clang's
Integrated Assembler did not handle .codeNN directives. Clang gained
support quite some time ago, so we can build stand/ with IAS.

In some cases there were small differences in generated object output.
In the case of gptzfsboot however using GNU as or Clang IAS to assemble
gptldr.S resulted in identical final gptzfsboot binary output.

Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D11115