sys/src/libc: fix ainc and adec (thanks Richard Miller)

all: import riscv and riscv64 support from /n/sources/contrib/miller/riscv.tar (2021-05-28) (thanks Richard Miller)

all: import mips64 and spim64 support from bitbucket.org/cherry9/4c (thanks Cherry Zhang)

all: import spim support from bitbucket.org/cherry9/plan9-loongson (thanks Cherry Zhang)

all: import power64 support from Charles Forsyth (thanks Charles Forsyth)

all: import amd64 from Bell Labs

sys/src/libc: add mixed-mode vlong/double (thanks Charles Forsyth)

sys/src/libc: add memory barrier in ainc and adec (thanks Richard Miller)

sys/src: times are unsigned (thanks Geoff Collyer)

sys/src/libc/fmt: use Rune instead of int in Convfmt (thanks Geoff Collyer)

sys/src/libc: prevent infinite loop in pow function (thanks Geoff Collyer)

sys/src/libc: replace sprint by snprint (thanks Geoff Collyer)

sys/src/libc: handle year >= 2100 in ctime (thanks Geoff Collyer)

sys/src/libc: update ARM atomic operations to work correctly on multiprocessors (thanks Richard Miller)

sys/src/libc: simplify nsec (thanks Charles Forsyth)

sys/src/libc: fix privalloc and privfree (thanks Charles Forsyth)

sys/src/libc: implement procsetname (thanks Charles Forsyth)

sys/src/libc: import leget and beget functions (thanks Francisco Ballesteros)

Plan 9 from Bell Labs Fourth Edition 2015-01-10

Plan 9 from Bell Labs Fourth Edition 2014-05-15

Plan 9 from Bell Labs Fourth Edition 2013-12-03

Plan 9 from Bell Labs Fourth Edition 2013-09-19

Plan 9 from Bell Labs Fourth Edition 2013-09-18

Plan 9 from Bell Labs Fourth Edition 2013-09-06

Plan 9 from Bell Labs Fourth Edition 2013-08-15