[gdb/testsuite] Fix gdb.xml/tdesc-regs.exp on riscv64

When running test-case gdb.xml/tdesc-regs.exp on riscv64-linux, I get:
...
(gdb) set tdesc file single-reg.xml^M
warning: Architecture rejected target-supplied description^M
(gdb) FAIL: gdb.xml/tdesc-regs.exp: set tdesc file single-reg.xml
UNSUPPORTED: gdb.xml/tdesc-regs.exp: register tests
...

The FAIL and UNSUPPORTED are produced here:
...
 # If no core registers were specified, assume this target does not
 # support target-defined registers.  Verify that we get a warning if
 # we try to use them.  This not only tests the warning, but also
 # reminds maintainers to add test support when they add the feature.

if {[string equal ${core-regs} ""]} {
    gdb_test "set tdesc file $single_reg_xml" \
	"warning: Target-supplied registers are not supported.*" \
	"set tdesc file single-reg.xml"
    unsupported "register tests"
    return 0
}
...

The test-case contains target-specific setting of the core-regs variable, and
adding this for riscv64 bypasses this code and makes the test-case pass.

However, without that change, the test-case shouldn't produce a FAIL since
gdb isn't doing anything wrong.

Fix this by producing instead:
...
PASS: $exp: set tdesc file single-reg.xml
UNSUPPORTED: $exp: register tests (missing architecture-specific core-regs setting)
...

Tested on riscv64-linux.

0 files changed, 0 insertions, 0 deletions