eld should warn/error when VMA/LMA is negative

lld does emit a diagnostic in this case. To maintain compatibility eld should emit an error/warning in this case.

```
cat > 1.t << \!
SECTIONS {
  .foo (-0x201) : AT(-0x201) { *(.text.foo) }
}
!

cat > 1.c << \!
int foo() { return 0; }
!

hexagon-clang -c 1.c  -ffunction-sections
hexagon-link 1.o -T 1.t
```

