[ELF] Rewrite R_RISCV_ALIGN nops when r.addend%4 != 0

For RVC, GNU assembler and LLVM integrated assembler add c.nop followed by a
sequence of 4-byte nops. Even if remove % 4 == 0, we have to split one 4-byte
nop and therefore need to write the code sequence, otherwise we create an
incorrect c.unimp.

(cherry picked from commit 78084d9)

Author: MaskRay

lld/ELF/Arch/RISCV.cpp

@@ -750,12 +750,13 @@ void elf::riscvFinalizeRelax(int passes) {
         p += size;
 
         // For R_RISCV_ALIGN, we will place `offset` in a location (among NOPs)
-        // to satisfy the alignment requirement. If `remove` is a multiple of 4,
-        // it is as if we have skipped some NOPs. Otherwise we are in the middle
-        // of a 4-byte NOP, and we need to rewrite the NOP sequence.
+        // to satisfy the alignment requirement. If both `remove` and r.addend
+        // are multiples of 4, it is as if we have skipped some NOPs. Otherwise
+        // we are in the middle of a 4-byte NOP, and we need to rewrite the NOP
+        // sequence.
         int64_t skip = 0;
         if (r.type == R_RISCV_ALIGN) {
-          if (remove % 4 != 0) {
+          if (remove % 4 || r.addend % 4) {
             skip = r.addend - remove;
             int64_t j = 0;
             for (; j + 4 <= skip; j += 4)

lld/test/ELF/riscv-relax-align-rvc.s

@@ -50,6 +50,12 @@
 # CHECK-NEXT:           c.addi  a0, 8
 # CHECK-EMPTY:
 
+# CHECK:      <.text2>:
+# CHECK-NEXT:           addi    a0, a1, 1
+# CHECK-NEXT:           c.addi  a0, 1
+# CHECK-NEXT:           c.nop
+# CHECK-NEXT:           c.addi  a0, 2
+
 .global _start
 _start:
   c.addi a0, 1
@@ -73,3 +79,10 @@ d:
   c.addi a0, 8
 .size d, . - d
 .size _start, . - _start
+
+.section .text2,"ax"
+.balign 16
+  addi a0, a1, 1
+  c.addi a0, 1
+.balign 8
+  c.addi a0, 2