gdb/riscv: Improved register alias name creation

gdb/riscv: Improved register alias name creation

This commit does two things:

 1. Makes use of the DECLARE_CSR_ALIAS definitions in riscv-opc.h to
 add additional aliases for CSRs.

 2. Only creates aliases for registers that are actually present on
 the target (as announced in the target XML description).

This means that the 'csr%d' aliases that exist will only be created
for those CSRs the target actually has, which is a nice improvement,
as accessing one of the CSRs that didn't exist would cause GDB to
crash with this error:

  valprint.c:1560: internal-error: bool maybe_negate_by_bytes(const gdb_byte*, unsigned int, bfd_endian, gdb::byte_vector*): Assertion `len > 0' failed.

When we look at the DECLARE_CSR_ALIAS lines in riscv-opc.h, these can
be split into three groups:

 DECLARE_CSR_ALIAS(misa, 0xf10, CSR_CLASS_I, PRIV_SPEC_CLASS_1P9, PRIV_SPEC_CLASS_1P9P1)

The 'misa' register used to exist of offset 0xf10, but was moved to
its current offset (0x301) in with privilege spec 1.9.1.  We don't
want GDB to create an alias called 'misa' as we will already have a
'misa' register created by the DECLARE_CSR(misa ....) call earlier in
riscv-opc.h

 DECLARE_CSR_ALIAS(ubadaddr, CSR_UTVAL, CSR_CLASS_I, PRIV_SPEC_CLASS_1P9, PRIV_SPEC_CLASS_1P10)
 DECLARE_CSR_ALIAS(sbadaddr, CSR_STVAL, CSR_CLASS_I, PRIV_SPEC_CLASS_1P9, PRIV_SPEC_CLASS_1P10)
 DECLARE_CSR_ALIAS(sptbr, CSR_SATP, CSR_CLASS_I, PRIV_SPEC_CLASS_1P9, PRIV_SPEC_CLASS_1P10)
 DECLARE_CSR_ALIAS(mbadaddr, CSR_MTVAL, CSR_CLASS_I, PRIV_SPEC_CLASS_1P9, PRIV_SPEC_CLASS_1P10)
 DECLARE_CSR_ALIAS(mucounteren, CSR_MCOUNTINHIBIT, CSR_CLASS_I, PRIV_SPEC_CLASS_1P9, PRIV_SPEC_CLASS_1P10)

These aliases are all CSRs that were removed in privilege spec 1.10,
and whose addresses were reused by new CSRs.  The names meaning of the
old names is totally different to the new CSRs that have taken their
place.  I don't believe we should add these as aliases into GDB.  If
the new CSR exists in the target then that should be enough.

 DECLARE_CSR_ALIAS(dscratch, CSR_DSCRATCH0, CSR_CLASS_I, PRIV_SPEC_CLASS_1P9, PRIV_SPEC_CLASS_1P11)

In privilege spec 1.11 the 'dscratch' register was renamed to
'dscratch0', however the meaning of the register didn't change.
Adding the 'dscratch' alias makes sense I think.

Looking then at the final PRIV_SPEC_CLASS_* field for each alias then
we can see that currently we only want to take the alias from
PRIV_SPEC_CLASS_1P11.  For now then this is what I'm using to filter
the aliases within GDB.

In the future there's no telling how DECLARE_CSR_ALIAS will be used.
I've heard it said that future RISC-V privilege specs will not reuse
CSR offsets again.  But it could happen.  We just don't know.

If / when it does we may need to revisit how aliases are created for
GDB, but for now this seems to be OK.

gdb/ChangeLog:

	* riscv-tdep.c (riscv_create_csr_aliases): Handle csr aliases from
	riscv-opc.h.
	(class riscv_pending_register_alias): New class.
	(riscv_check_tdesc_feature): Take vector of pending aliases and
	populate it as appropriate.
	(riscv_setup_register_aliases): Delete.
	(riscv_gdbarch_init): Create vector of pending aliases and pass it
	to riscv_check_tdesc_feature in all cases.  Use the vector to
	create the register aliases.

gdb/testsuite/ChangeLog:

	* gdb.arch/riscv-tdesc-regs-32.xml: New file.
	* gdb.arch/riscv-tdesc-regs-64.xml: New file.
	* gdb.arch/riscv-tdesc-regs.c: New file.
	* gdb.arch/riscv-tdesc-regs.exp: New file.