EF_RISCV_N32: Add a bit definition in e_flags

EF_RISCV_N32 is for ILP32* ABI on RV64* ISA. It use ELF-32 Object
File and needs a flag-bit in e_flags field.

Signed-off-by: Liao Shihua <shihua@iscas.ac.cn>
Signed-off-by: Guo Ren <guoren@linux.alibaba.com>

riscv-elf.adoc

@@ -277,12 +277,12 @@ below.
 +
 [[e-flags-layout]]
 .Layout of e_flags
-[cols="1,2,1,1,3,5"]
+[cols="1,2,1,1,1,3,5"]
 [width=80%]
 |===
-| Bit 0 | Bits 1 - 2 | Bit 3 | Bit 4 | Bits 5 - 23 | Bits 24 - 31
+| Bit 0 | Bits 1 - 2 | Bit 3 | Bit 4 | Bit 5 | Bits 6 - 23 | Bits 24 - 31
 
-| RVC   | Float ABI  | RVE   | TSO   | *Reserved*  | *Non-standard extensions*
+| RVC   | Float ABI  | RVE   | TSO   | N32   | *Reserved*  | *Non-standard extensions*
 |===
 
 +
@@ -320,6 +320,9 @@ below.
   EF_RISCV_TSO (0x0010)::: This bit is set when the binary requires the RVTSO
   memory consistency model.
 
+  EF_RISCV_N32 (0x0020)::: This bit is set when the binary requires the ILP32*
+  ABI on RV64* ISAs.
+
 Until such a time that the *Reserved* bits (0x00ffffe0) are allocated by future
 versions of this specification, they shall not be set by standard software.
 Non-standard extensions are free to use bits 24-31 for any purpose. This may
@@ -355,6 +358,9 @@ raise an error.
   TSO::: Input files can have different values for the TSO field; the linker
   should set this field if any of the input objects have the TSO field set.
 
+  N32::: Linker should report errors if object files of different value
+  for N32 field.
+
 NOTE: The static linker may ignore the compatibility checks if all fields in the
 `e_flags` are zero and all sections in the input file are non-executable
 sections.