libi386: remove CLANG_NO_IAS workaround

Clang's Integrated Assembler was previously disabled for i386 with the
note that it "doesn't grok .codeNN directives yet." This is no longer
the case (and hasn't been for some time), and the assembled output .text
is identical between gas and IAS.

MFC after: 2 months
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 188b0da1ce6708f4168309f5b925f1c9df836bc7)