x86: Extend assembler to generate GNU property notes
Add -mx86-used-note=[yes|no] option to generate (or not) GNU property
notes with GNU_PROPERTY_X86_FEATURE_2_USED and GNU_PROPERTY_X86_ISA_1_USED
properties. If the assembly input contains no instructions, set the
GNU_PROPERTY_X86_UINT32_VALID bit in GNU_PROPERTY_X86_FEATURE_2_USED
property. Add a --enable-x86-used-note configure time option to set the
default behavior. Set the default if the configure option is not used
to "no".