New attribute: Tag_RISCV_reserved_register

- Motivation:

  Tag_RISCV_reserved_register used for record reserved register information,
  desiged for resolve checking compatibility between ABI subvariants like
  overlay ABI.

- Meaning:

  This attribute is a bit-vector of reserved register, syntax for
  assembly language please refer
  https://github.com/riscv-non-isa/riscv-asm-manual .

- Merge Rule:

  It will report errors if link object files with different
  Tag_RISCV_reserved_register values, but allowed link with object with and
  without Tag_RISCV_reserved_register value, the final value will take from the
  object which has set Tag_RISCV_reserved_register.

Author: kito-cheng

riscv-elf.adoc

@@ -896,6 +896,7 @@ value if the tag number is even.
 | Tag_RISCV_priv_spec                 |        8 | uleb128        | Indicates the major version of the privileged specification.
 | Tag_RISCV_priv_spec_minor           |       10 | uleb128        | Indicates the minor version of the privileged specification.
 | Tag_RISCV_priv_spec_revision        |       12 | uleb128        | Indicates the revision version of the privileged specification.
+| Tag_RISCV_reserved_register         |       14 | uleb128        | Indicates the extra reserved register information.
 | Reserved for non-standard attribute | >= 32768 | -              | -
 |===
 
@@ -942,6 +943,23 @@ Tag_RISCV_priv_spec contains the major/minor/revision version information of
 the privileged specification. It will report errors if object files of different
 privileged specification versions are merged.
 
+===== Tag_RISCV_reserved_register, 14, uleb128=register bit-vector
+
+Tag_RISCV_reserved_register contains the information about the reserved
+registers which are not reserved by ABI, it might reserved by specific
+compiler option like `-ffixed-x4` or special purpose ABI variant like
+overlay ABI.
+
+Format of Tag_RISCV_reserved_register is a bit-vector, each bit corresponding to
+a register, the bit order is same as dwarf register number.
+First bit is must set, used to distinguish between attribute is unset or having
+empty reserved register set.
+
+It will report errors if link object files with different
+Tag_RISCV_reserved_register values, but allowed link with object with and
+without Tag_RISCV_reserved_register value, the final value will take from the
+object which has set Tag_RISCV_reserved_register.
+
 == Code relaxation
 
 At link time, when all the memory objects have been resolved, the code sequence