RISC-V: Support new .option arch directive.

https://github.com/riscv/riscv-asm-manual/pull/67

Format:
.option arch, +<extension><version>, ...
.option arch, -<extension>
.option arch, =<ISA string>

The new direcitve is used to enable/disable extensions for the specific
code region.  For example,

.attribute arch, "rv64ic"   # arch = rv64i2p0_c2p0
.option push
.option arch, +d2p0, -c     # arch = rv64i2p0_f2p0_d2p0, f is added implied
.option arch, =rv32gc       # arch = rv32i2p0_m2p0_a2p0_f2p0_d2p0_c2p0
.option pop                 # arch = rv64i2p0_c2p0

Note that,
1. ".option rvc/norvc" have the same behavior as ".option arch +c/-c".
2. ".option arch -i" is illegal, since we cannot remove base i extension.
3. If arch=rv64i2p0, then ".option arch, +i3p0" will update the i's version
   from 2.0 to 3.0.
4. If arch=rv64i3p0, then ".option arch, +i" will update the i's version
   from 2.0 to the default one according to the chosen isa spec.

bfd/
	* elfxx-riscv.c (riscv_add_subset): If the subset is already added,
	and the new versions are not RISCV_UNKNOWN_VERSION, then update the
	versions to the subset list.
	(riscv_copy_subset): New function.  Copy the subset from list.
	(riscv_copy_subset_list): New function.  Return the new copyed list.
	(riscv_update_subset): Updated to make .option arch directives workable.
	* elfxx-riscv.h: Updated.
gas/
	* config/tc-riscv.c (riscv_subsets): Defined as a pointer.
	(riscv_rps_as): Init the subset_list to NULL, we will set it later
	once riscv_opts_stack is created or updated.
	(struct riscv_option_stack, riscv_opts_stack): Moved forward.
	(riscv_set_arch): Updated.
	(s_riscv_option): Support new .option arch directive, to add, remove
	or update subsets for the specific code region.
	(riscv_write_out_attrs): Updated.
	* doc/c-riscv.texi: Added document for new .option arch directive.
	* testsuite/gas/riscv/option-arch-01a.d: New testcase.
	* testsuite/gas/riscv/option-arch-01b.d: Likewise.
	* testsuite/gas/riscv/option-arch-01.s: Likewise..
	* testsuite/gas/riscv/option-arch-02.d: Likewise.
	* testsuite/gas/riscv/option-arch-02.s: Likewise.
	* testsuite/gas/riscv/option-arch-fail.d: Likewise.
	* testsuite/gas/riscv/option-arch-fail.l: Likewise.
	* testsuite/gas/riscv/option-arch-fail.s: Likewise.

1 files changed, 14 insertions, 1 deletions

diff --git a/gas/doc/c-riscv.texi b/gas/doc/c-riscv.texi
index bfbf61d..697be3a 100644
--- a/gas/doc/c-riscv.texi
+++ b/gas/doc/c-riscv.texi
@@ -194,7 +194,7 @@ command-line options are respected for the bulk of the file being assembled.
 @itemx norvc
 Enables or disables the generation of compressed instructions.  Instructions
 are opportunistically compressed by the RISC-V assembler when possible, but
-sometimes this behavior is not desirable.
+sometimes this behavior is not desirable, especially when handling alignments.
 
 @item pic
 @itemx nopic
@@ -212,6 +212,19 @@ desirable.
 @itemx no-csr-check
 Enables or disables the CSR checking.
 
+@item arch, @var{+extension[version]} [,...,@var{+extension_n[version_n]}]
+@itemx arch, @var{-extension} [,...,@var{-extension_n}]
+@itemx arch, @var{=ISA}
+Enables or disables the extensions for specific code region.  For example,
+@samp{.option arch, +m2p0} means add m extension with version 2.0, and
+@samp{.option arch, -f, -d} means remove extensions, f and d, from the
+architecture string.  Note that, @samp{.option arch, +c, -c} have the same
+behavior as @samp{.option rvc, norvc}.  However, they are also undesirable
+sometimes.  Besides, @samp{.option arch, -i} is illegal, since we cannot
+remove the base i extension anytime.  If you want to reset the whole ISA
+string, you can also use @samp{.option arch, =rv32imac} to overwrite the
+previous settings.
+
 @cindex INSN directives
 @item .insn @var{type}, @var{operand} [,...,@var{operand_n}]
 @itemx .insn @var{insn_length}, @var{value}