Move some llvm-mc tests where they belong

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@353211 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/test/Assembler/empty-string.s b/test/MC/RISCV/empty-string.s
similarity index 100%
rename from test/Assembler/empty-string.s
rename to test/MC/RISCV/empty-string.s

diff --git a/test/Assembler/return-column.s b/test/MC/X86/return-column.s
similarity index 100%
rename from test/Assembler/return-column.s
rename to test/MC/X86/return-column.s