RFC: Linux gABI: Add a GNU_PROPERTY_BY_LINKER property

["H.J. Lu" <hjl dot tools at gmail dot com>]

Linkers group input note sections with the same name into one output
note section with the same name.  One output note section is placed in
one PT_NOTE segment.  Linkers with GNU property support merge all input
.note.gnu.property sections into one output .note.gnu.property section
with a single NT_GNU_PROPERTY_TYPE_0 note in a single PT_NOTE segment.
Since linkers without GNU property support treat input .note.gnu.property
section as a generic note section and just concatenate all input
.note.gnu.property sections into one output .note.gnu.property section
without merging them, we may see one or more NT_GNU_PROPERTY_TYPE_0 notes
in PT_NOTE segment, which is invalid.

GNU_PROPERTY_X86_UINT32_VALID was defined to address this issue such that
linker sets the bit in values of x86 properties for non-relocatable
outputs.  But it isn't sufficient:

1. It doesn't cover generic properties.
2. When -mx86-used-note=yes is passed to x86 assembler, the
GNU_PROPERTY_X86_UINT32_VALID bit is set in GNU_PROPERTY_X86_ISA_1_USED
property in object file and linkers without GNU property support generate
invalid NT_GNU_PROPERTY_TYPE_0 notes with the GNU_PROPERTY_X86_UINT32_VALID
bit set.

I am proposing the following changes:

1. Add a GNU_PROPERTY_BY_LINKER property which should only be set by
linker for non-relocatable outputs to indicate the property note is
valid and generated by new linkers.  Loaders can check this property
to verify that the property note is valid.
2. Remove GNU_PROPERTY_X86_UINT32_VALID.
3. Define GNU_PROPERTY_X86_ISA_1_BASE for GNU_PROPERTY_X86_ISA_1_USED,
which has the same bit as GNU_PROPERTY_X86_UINT32_VALID and use it
for -mx86-used-note=yes with x86 assembler.

-- 
H.J.