Log message:
wasi-libc: Disable SSP explicitly and disable stack check too

Fix build when PKGSRC_USE_SSP=all and PKGSRC_USE_STACK_CHECK=yes
are defined. Reported by wiz@.

Log message:
wasi-libc: set CLANGBASE instead of PKG_CC/CXX

Otherwise it breaks when user has CLANGBASE=/usr in mk.conf.
Also set PKG_FAIL_REASON when llvm doesn't have llvm-target-webassembly.

Log message:
lang/wasi-libc: import wasi-libc-0.0.0pre20210923

WASI Libc is a libc for WebAssembly programs built on top of WASI
system calls. It provides a wide array of POSIX-compatible C APIs,
including support for standard I/O, file I/O, filesystem manipulation,
memory management, time, string, environment variables, program
startup, and many other APIs.

WASI Libc is sufficiently stable and usable for many purposes, as
most of the POSIX-compatible APIs are stable, though it is continuing
to evolve to better align with wasm and WASI.