arch-riscv,misc: Added M5_VAR_USED to MiscRegNames
arch-riscv,misc: Added M5_VAR_USED to MiscRegNames

Clang compilers return an error about MiscRegNames being unused.
M5_VAR_USED fixes this.

Change-Id: I515c5d1e8837020b674de49039c0525f896b7e37
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/29452
Reviewed-by: Jason Lowe-Power <92de577f37050ad05b60626d2f17913f40dcb1a2@gmail.com>
Maintainer: Jason Lowe-Power <92de577f37050ad05b60626d2f17913f40dcb1a2@gmail.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

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