stand: remove CLANG_NO_IAS from zfsldr

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). For zfsldr I compared objdump
output between GNU as- and Clang IAS-built zfsldr and .text was
identical (changes were limited to the object's ELF headers and debug
info).

Sponsored by: The FreeBSD Foundation

(cherry picked from commit 698194ab249ffd23422048dc2157042fd5442288)