[llvm] [llvm-objdump][RISCV] Use ISA mapping symbols for per-region disassembly (PR #193448)

[Sam Elliott via llvm-commits]

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@@ -0,0 +1,30 @@
+## Objects produced by tools predating "$x<ISA>" mapping-symbol emission have
+## no such symbols in their .symtab.  llvm-objdump must fall back to the
+## file-level feature set derived from Tag_RISCV_arch (the pre-PR behaviour)
+## and decode V-family encodings using those features.
+##
+## This cannot be assembled because modern llvm-mc always emits an initial
+## "$xrv64i2p1" mapping symbol; yaml2obj lets us handcraft the legacy layout.
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lenary wrote:

Do we have a test for the first mapping symbol just being `$x`? Maybe this is equivalent.

https://github.com/llvm/llvm-project/pull/193448