druntime: Treat WASI as Posix

[QuantumSegfault]

Adds WASI (or rather wasi-libc) support to core.sys.posix, and makes all WASI triples define Posix.

Also removes or adjust other WASI specific code accordingly.

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Upstreaming of ldc-developers/ldc#5230

[QuantumSegfault]

@kinke
@thewilsonator
@denizzzka

[github-actions]

DMD perf check

Metric	Base	PR	delta
compile hello.d (instr)	215.6 M	216.1 M	+0.21%
compile hello.d -O (instr)	234.1 M	234.6 M	+0.19%
compile Phobos (instr)	5,075.3 M	5,075.7 M	+0.01%
dmd binary size (stripped)	6.88 MB	6.88 MB	0.00%
hello binary size	0.72 MB	0.72 MB	0.00%
peak RSS (compile hello.d)	44 MB	44 MB	-0.15%
peak RSS (compile Phobos)	633 MB	633 MB	-0.02%

[rikkimax]

Append with empty changelist, and push.
Looks like CircleCI is just being weird.