bfc: tiny handwritten brainfuck compiler

Featuring handwritten ELF headers and no optimiztions whatsoever.

Currently at 848 bytes, of which 248 bytes are the headers.

Biggest size wins:

commit	change	before	after	delta
6b41b97	reuse registers between syscalls; use xor to zero a register	1263	1089	-174
dd0b0f5	more xor x,x instead of mov x,0	1089	1070	-19
c6c27bc	replace mov r64, imm with push imm; pop r64 where shorter	1062	885	-177
5bb9793	write JE with placeholder	882	866	-16