[Test][JITLink] Correctly generate the R_X86_64_PC8 relocation. (#109283)

Previously, ELF_R_X86_64_PC8.s doesn't produce the R_X86_64_PC8
relocation. This patch helps fix it by emitting a byte `main - .` to the
.rodata section.

Author: higuoxing

llvm/test/ExecutionEngine/JITLink/x86-64/ELF_R_X86_64_PC8.s

@@ -12,8 +12,5 @@ main:
 	retq
 	.size	main, .-main
 
-	.type	P,@object
-	.globl	P
-P:
-	.byte main-. # Generate R_X86_64_PC8 relocation.
-  .size P, .-P
+	.rodata
+	.byte	main-. # Generate R_X86_64_PC8 relocation.