cmd/asm: inconsistent handling of explicit zero offset for TEXT pseudo

`cmd/asm` in Go 1.12.1 and before willingly assembles
```
TEXT ·MyFunc+0(SB), $0
```
However, `cmd/asm` in `-gensymabis` mode doesn't recognize this line as a definition. That leads to esoteric symbol ABI errors during linking on Go 1.12.

Either offset for the `TEXT` pseudo should be fully banned or `-gensymabis` should handle explicit zero offsets.