[sanitizer_common] Fix signal_line.cpp on SPARC (#100535)

[sanitizer_common] Fix signal_line.cpp on SPARC (#100535)

```
  SanitizerCommon-ubsan-sparc-Linux :: Linux/signal_line.cpp
```
currently `FAIL`s on Linux/sparc64 (32 and 64-bit) for `n == 2`. Instead
of the expected `SIGSEGV`, the test dies with `SIGBUS`. `strace` reveals
that this is due to a unaligned access:
```
--- SIGBUS {si_signo=SIGBUS, si_code=BUS_ADRALN, si_addr=0x1} ---
```
which is to be expected on a strict-alignment target like SPARC. Fixed
by changing the invalid pointer to be better aligned.

Tested on `sparc64-unknown-linux-gnu` and `x86_64-pc-linux-gnu`.

(cherry picked from commit 94394ca980f8ecbd845155d2170cfd865e4d62dc)