RISC-V: add testcase to check line number emission for .insn

Since no such test looks to exist, derive one from insn.s.

gas/testsuite/gas/riscv/insn-dwarf.d

@@ -0,0 +1,71 @@
+#as: -march=rv64ifc -gdwarf-2
+#name: Dwarf line number info for .insn
+#objdump: -WL -w
+#source: insn.s
+
+.*:[ 	]+file format .*
+
+Contents of the .debug_line section:
+
+CU: .*/insn.s:
+File name +Line number +Starting address.*
+insn.s +2 +0.*
+insn.s +3 +0x4.*
+insn.s +4 +0x8.*
+insn.s +5 +0xc.*
+insn.s +6 +0x10.*
+insn.s +7 +0x14.*
+insn.s +8 +0x18.*
+insn.s +9 +0x1c.*
+insn.s +10 +0x20.*
+insn.s +11 +0x24.*
+insn.s +13 +0x28.*
+insn.s +14 +0x2a.*
+insn.s +15 +0x2c.*
+insn.s +16 +0x2e.*
+insn.s +17 +0x30.*
+insn.s +18 +0x32.*
+insn.s +19 +0x34.*
+insn.s +20 +0x36.*
+insn.s +22 +0x38.*
+insn.s +23 +0x3c.*
+insn.s +24 +0x40.*
+insn.s +25 +0x44.*
+insn.s +26 +0x48.*
+insn.s +27 +0x4c.*
+insn.s +28 +0x50.*
+insn.s +29 +0x54.*
+insn.s +30 +0x58.*
+insn.s +31 +0x5c.*
+insn.s +33 +0x60.*
+insn.s +34 +0x62.*
+insn.s +35 +0x64.*
+insn.s +36 +0x66.*
+insn.s +37 +0x68.*
+insn.s +38 +0x6a.*
+insn.s +39 +0x6c.*
+insn.s +40 +0x6e.*
+insn.s +41 +0x70.*
+insn.s +43 +0x72.*
+insn.s +44 +0x76.*
+insn.s +45 +0x7a.*
+insn.s +46 +0x7e.*
+insn.s +47 +0x82.*
+insn.s +48 +0x86.*
+insn.s +49 +0x8a.*
+insn.s +50 +0x8e.*
+insn.s +51 +0x92.*
+insn.s +52 +0x96.*
+insn.s +53 +0x9a.*
+insn.s +54 +0x9e.*
+insn.s +55 +0xa2.*
+insn.s +57 +0xa6.*
+insn.s +58 +0xa8.*
+insn.s +59 +0xac.*
+insn.s +60 +0xb2.*
+insn.s +61 +0xba.*
+insn.s +62 +0xbc.*
+insn.s +63 +0xc0.*
+insn.s +64 +0xc6.*
+insn.s +- +0xce
+#pass