Log message:
wasi-libc: fix build on SunOS

Log message:
Fix packages that use GITSUB_MODULES on SunOS and possibly others

This is annoying, but for now we must always explicitly combine
GITHUB_SUBMODULES with EXTRACT_USING+=bsdtar.

This is because mk/fetch/github.mk uses OPTS_TAR=--strip-components=1
and that is not supported by nbtar(pax), which is the default pkgsrc
tar on some platforms. We cannot override EXTRACT_USING in github.mk
because that is too late.

We should switch all platforms to bsdtar and retire pax.

Log message:
wasi-*: eliminate -march and/or -mcpu CFLAGS user might have set

Random CFLAGS from mk.conf are not a good idea in a cross compile scenario.
Also reset ABI to empty string since -m32 can break some native CMake tests.

Log message:
wasi-*: force ABI=32

These packages were failing when ABI=64 in mk.conf, because then -m64 is
passed down to clang and WebAssembly currently only supports -m32.

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.