[lld][AVR] Allow R_AVR_13_PCREL to overflow

This matches the behavior of avr-ld, and is needed for devices like the
ATtiny85 which only have rjmp/rcall (no jmp/call) but have 8kB of flash
memory. Without this change, jumps over 4kB would not be possible on the
ATtiny85.

Author: aykevl

lld/ELF/Arch/AVR.cpp

@@ -238,7 +238,6 @@ void AVR::relocate(uint8_t *loc, const Relocation &rel, uint64_t val) const {
     break;
   }
   case R_AVR_13_PCREL: {
-    checkInt(loc, val - 2, 13, rel);
     checkAlignment(loc, val, 2, rel);
     const uint16_t target = (val - 2) >> 1;
     write16le(loc, (read16le(loc) & 0xf000) | (target & 0xfff));

lld/test/ELF/avr-reloc-error.s

@@ -34,7 +34,6 @@ brlt callee2
 __start:
 
 # PCREL13-NOT: callee0
-# PCREL13:     error: {{.*}} relocation R_AVR_13_PCREL out of range: {{.*}} is not in [-4096, 4095]; references 'callee1'
 # PCREL13:     error: {{.*}} improper alignment for relocation R_AVR_13_PCREL: {{.*}} is not aligned to 2 bytes
 rjmp  callee0
 rcall callee1

lld/test/ELF/avr-reloc.s

@@ -82,6 +82,10 @@ sbic  b, 1    ; R_AVR_PORT5
 ; CHECK-NEXT:  rjmp .-36
 ; CHECK-NEXT:  breq .+26
 ; CHECK-NEXT:  breq .-40
+; CHECK-NEXT:  rjmp .-4096
+; CHECK-NEXT:  rjmp .+4094
+; CHECK-NEXT:  rjmp .+4094
+; CHECK-NEXT:  rjmp .-4096
 ; CHECK-NEXT:  breq .-128
 ; CHECK-NEXT:  breq .+126
 ; HEX-LABEL:   section .PCREL:
@@ -91,6 +95,10 @@ rjmp foo + 32  ; R_AVR_13_PCREL
 rjmp foo - 32  ; R_AVR_13_PCREL
 breq foo + 32  ; R_AVR_7_PCREL
 breq foo - 32  ; R_AVR_7_PCREL
+rjmp 1f - 4096  $ 1:  ; R_AVR_13_PCREL
+rjmp 1f + 4094  $ 1:  ; R_AVR_13_PCREL
+rjmp 1f - 4098  $ 1:  ; R_AVR_13_PCREL (overflow)
+rjmp 1f + 4096  $ 1:  ; R_AVR_13_PCREL (overflow)
 breq 1f - 128   $ 1:  ; R_AVR_7_PCREL
 breq 1f + 126   $ 1:  ; R_AVR_7_PCREL