Smaller C: fixes, improvements

Fixes/workarounds:
- octal escape sequence is to be terminated no later than after three
octal digits:
"\1011" is now parsed equivalently to "A1"
- workaround truncation of immediate operands to 16 bits in MIPS
instructions addi[u], subi[u], slti[u], andi, ori, xori in RetroBSD's as
(disabled since as is being fixed).

Improvements:
- warn on string literal truncation when initializing array of char
- generate trap/break-less form of MIPS div[u]

New:
- experimental support for __interrupt functions (x86/huge only)