p2: RPATH needs no subst, but LDDLLFLAGS does

we got rid of RDLLFLAGS, so use LDDLLFLAGS (subst only for darwin -install_name)
fix LIBP2A_AWAY/LIBP2A_BACK
p2 jit: fix boolean PN_STR0 test

potion_x86_test_asm, potion_x86_testjmp, potion_x86_notjmp: improve docs,
use proper label names, use word-size PN_STR0 checks, not only the last byte.

TODO: we also need to check for empty tuple, hash, bytes
p2 doc: UCONTEXT.txt yapcna_2013.pod (not used, but useful)