cmd/link/internal/loong64: use BREAK 0 as the code pad sequence

As the comment on CodePad goes, we "might want to pad with a trap
instruction to catch wayward programs". The current behavior of
zero-padding is equivalent to padding with an instruction of 0x00000000,
which is invalid according to the LoongArch manuals nevertheless, but
rumor has it that some early and/or engineering samples of Loongson
3A5000 recognized it (maybe behaving like NOP). It is better to avoid
undocumented behavior and ensure execution flow would not overflow the
pads.

Change-Id: I531b1eabeb355e9ad4a2d5340e61f2fe71349297
Reviewed-on: https://go-review.googlesource.com/c/go/+/475616
Reviewed-by: abner chenc <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
Run-TryBot: Cherry Mui <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>

Author: xen0n

src/cmd/link/internal/loong64/obj.go

@@ -19,6 +19,7 @@ func Init() (*sys.Arch, ld.Arch) {
 		Minalign:         minAlign,
 		Dwarfregsp:       dwarfRegSP,
 		Dwarfreglr:       dwarfRegLR,
+		CodePad:          []byte{0x00, 0x00, 0x2a, 0x00}, // BREAK 0
 		Adddynrel:        adddynrel,
 		Archinit:         archinit,
 		Archreloc:        archreloc,