[JITLink][RISCV] Add feature relax for addsub tests. NFC (llvm#121204)

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 \