scons/windows: Support build with LLVM 9.

As X86AsmPrinter component is gone, LLVMX86AsmPrinter got replaced
with LLVMRemarks, LLVMBitstreamReader and LLVMDebugInfoDWARF.

Tests done with llvm-config on both LLVM 8 and 9 indicate that
mcjit, bitwriter and x86asmprinter fully fit inside engine component.

On other platforms and with meson build mcdisassembler was used to replace
X86AsmPrinter but mcdisassembler also fully fits inside engine component
for LLVM>=8 according to same tests.

v2: Avoid duplicating code related to Mingw pthreads.

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>

Cc: 19.1 19.2 <mesa-stable@lists.freedesktop.org>

On 19.1 this patch does not apply cleanly without 88eb2a1f
22 jobs for !1894 with windows-llvm9 in 14 minutes and 31 seconds (queued for 2 seconds)
merge request