stand: remove CLANG_NO_IAS from pxeldr

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. In most cases IAS- and GNU as-assembled boot components were
identical, and CLANG_NO_IAS was already removed from other components.

Clang IAS produces different output for some components, including
pxeldr, so CLANG_NO_IAS was not previously removed for those.

In the case of pxeldr the difference is that IAS adds a size override
prefix (67h) to three instructions to specify a 32-bit address, even
though the two high bytes are zero and the address fits in 16 bits.
this wastes three bytes per instruction and causes some additional nop
npadding to be required elsewhere in the object, but pxeboot is not
size-constrained so it doesn't matter.

Sponsored by: The FreeBSD Foundation

(cherry picked from commit aa2dd958b489d590be9573f1bd05ec5b8a12f016)