We already check that jump over 300 bytes gap is 3 bytes in code16-mode.
Some clang versions generate 3-byte opcode for short jumps which makes
boot.img blow over 512-byte limit. Enforce -no-integrated-as in such cases

There are no $t4 or $t5 in N32 but there are $a4 and $a5.

These platforms don't have a hard limit on size of resulting code16
code, so we don't care if assembly is bigger than necessarry.

Previously we relied on assumption that clang always needs -no-integrated-as
but it's not always true.