[toolchain] lib/clan/llvm.build.mk: Shouldn't BUILD_TRIPLE definition rely host 'cc -dumpmachine'?

[Gerald Pfeifer]

On Sun, 29 Oct 2017, Eddy Petrișor wrote:
> Yep --and it is even more complicated: gcc vs. clang are sometimes 
> different for the target listed. . .
> 
> For example -m32 for amd64 changes the clang result:
> 
> # clang -dumpmachine
> x86_64-unknown-freebsd12.0
> 
> ..
> 
> # gcc7 -dumpmachine
> x86_64-portbld-freebsd12.0

That's not actually related to GCC, but the lang/gcc* ports using
the FreeBSD Ports Collection's default that explicitly set

  CONFIGURE_TARGET?=  ${ARCH}-portbld-${OPSYS:tl}${OSREL}

By default GCC would use the same as clang.

Gerald