[JITLink][RISCV] Add feature relax for addsub tests. NFC

R_RISCV_{ADD*/SUB*} relocations are kept only when feature relax
enabled. So it is better to add relax to the test, so that
relocs can be reserved for processing by the jitlink. That's
what this test really wants to test.

Author: zhaoqi5

llvm/test/ExecutionEngine/JITLink/RISCV/riscv_reloc_add.s renamed to llvm/test/ExecutionEngine/JITLink/RISCV/ELF_reloc_add.s

@@ -1,6 +1,8 @@
 # RUN: rm -rf %t && mkdir -p %t
-# RUN: llvm-mc -triple=riscv64 -filetype=obj -o %t/riscv64_reloc_add.o %s
-# RUN: llvm-mc -triple=riscv32 -filetype=obj -o %t/riscv32_reloc_add.o %s
+# RUN: llvm-mc -triple=riscv64 -mattr=+relax -filetype=obj \
+# RUN:     -o %t/riscv64_reloc_add.o %s
+# RUN: llvm-mc -triple=riscv32 -mattr=+relax -filetype=obj \
+# RUN:     -o %t/riscv32_reloc_add.o %s
 # RUN: llvm-jitlink -noexec -check %s %t/riscv64_reloc_add.o \
 # RUN:     -slab-allocate=1Mb -slab-address=0x1000 -slab-page-size=0x1000
 # RUN: llvm-jitlink -noexec -check %s %t/riscv32_reloc_add.o \