cmake-utils.eclass: Make the new ASM-ATT rules actually work

cmake-utils.eclass: Make the new ASM-ATT rules actually work

The previous attempt actually broke ASM in media-libs/vulkan-loader
entirely so that it fell back to C code. After much experimentation
and combing through strace output, I found that -x assembler is needed
to handle non-standard file extentions and linking is done as a
separate step. CMAKE_ASM-ATT_LINK_FLAGS therefore needs to be defined
with -nostdlib to avoid errors about undefined main symbols.

Closes: https://bugs.gentoo.org/625844