[lld] Fill .text section gaps with INT3 only on x86 targets.

It doesn't make sense on ARM and using default 0 fill is compatible
with MSVC.

(It's more noticeable ARM64EC targets, where additional padding mixed
with alignment is used for entry thunk association, so there are more
gaps).

Reviewed By: mstorsjo

Differential Revision: https://reviews.llvm.org/D145962