RISC-V: Add description for -march-attr/-mno-arch-attr options in gas doc.

gas/
	* doc/c-riscv.texi: Add the doc entries for -march-attr/
	-mno-arch-attr command line options.

gas/ChangeLog

@@ -1,3 +1,8 @@
+2020-02-19  Nelson Chu  <nelson.chu@sifive.com>
+
+	* doc/c-riscv.texi: Add the doc entries for -march-attr/
+	-mno-arch-attr command line options.
+
 2020-02-19  Nelson Chu  <nelson.chu@sifive.com>
 
 	* testsuite/gas/riscv/c-add-addi.d: New testcase.

gas/doc/c-riscv.texi

@@ -59,6 +59,20 @@ required to materialize symbol addresses. (default)
 @item -mno-relax
 Don't do linker relaxations.
 
+@cindex @samp{-march-attr} option, RISC-V
+@item -march-attr
+Generate the default contents for the riscv elf attribute section if the
+.attribute directives are not set.  This section is used to record the
+information that a linker or runtime loader needs to check compatibility.
+This information includes ISA string, stack alignment requirement, unaligned
+memory accesses, and the major, minor and revision version of privileged
+specification.
+
+@cindex @samp{-mno-arch-attr} option, RISC-V
+@item -mno-arch-attr
+Don't generate the default riscv elf attribute section if the .attribute
+directives are not set.
+
 @end table
 @c man end