[RISCV] Add a test with unknown argument of .attribute directive

Author: s-barannikov

llvm/test/MC/RISCV/invalid-attribute.s

@@ -1,11 +1,15 @@
 ## Negative tests:
+##  - Unknown attribute name.
 ##  - Feed integer value to string type attribute.
 ##  - Feed string value to integer type attribute.
 ##  - Invalid arch string.
 
 # RUN: not llvm-mc %s -triple=riscv32 -filetype=asm 2>&1 | FileCheck %s
 # RUN: not llvm-mc %s -triple=riscv64 -filetype=asm 2>&1 | FileCheck %s
 
+.attribute unknown, "unknown"
+# CHECK: [[@LINE-1]]:12: error: attribute name not recognised: unknown
+
 .attribute arch, "foo"
 # CHECK: [[@LINE-1]]:18: error: invalid arch name 'foo', string must begin with rv32{i,e,g} or rv64{i,e,g}