stand: remove CLANG_NO_IAS from btx and gptboot

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

Note that in some cases there are small differences in the generated
output, so CLANG_NO_IAS should be removed only after testing (or after
finding no differences in the output).

PR: 205250, 233094
Sponsored by: The FreeBSD Foundation