[SDKS, LLVM] Disable detection and use of libxml2 (#11340)

[SDKS, LLVM] Disable detection and use of libxml2 (#11340)

Mono doesn't use libxml2 and neither do the LLVM bits used by Mono when building
the AOT cross-compilers. Presence of `-lxml2` on the command line breaks 32-bit
builds on 64-bit Linux hosts:

    libtool: link: cc -shared  -fPIC -DPIC  -Wl,--whole-archive ./.libs/libmini.a ./.libs/libmono-ee-interp.a ./.libs/libmono-dbg.a ../../mono/metadata/.libs/libmonoruntimesgen.a ../../mono/sgen/.libs/libmonosgen.a ../../mono/utils/.libs/libmonoutils.a ../../mono/eglib/.libs/libeglib.a -Wl,--no-whole-archive  -lLLVMBitWriter -lLLVMAnalysis -lLLVMProfileData -lLLVMObject -lLLVMMCParser -lLLVMMC -lLLVMBitReader -lLLVMCore -lLLVMBinaryFormat -lLLVMSupport -lLLVMDemangle -L$HOME/xamarin-android/external/mono/sdks/out/llvm-llvm32/lib -lz -ltinfo -lxml2 -lstdc++ -lm -lrt -ldl -lpthread  -O0 -ggdb3 -m32 -g -mno-tls-direct-seg-refs -Wl,-export-dynamic   -Wl,-soname -Wl,libmonosgen-2.0.so.1 -o .libs/libmonosgen-2.0.so.1.0.0
    mv -f .deps/libmini_la-aot-runtime-wasm.Tpo .deps/libmini_la-aot-runtime-wasm.Plo
    /usr/bin/ld: skipping incompatible //usr/lib/x86_64-linux-gnu/libxml2.so when searching for -lxml2
    /usr/bin/ld: skipping incompatible //usr/lib/x86_64-linux-gnu/libxml2.a when searching for -lxml2
    /usr/bin/ld: cannot find -lxml2
    collect2: error: ld returned 1 exit status
    make[7]: *** [Makefile:1495: libmonosgen-2.0.la] Error 1

Installation of `libxml2-dev` (on Ubuntu/Debian + friends) for the x86
architecture (or x86_64 architecture on 32-bit host) would be possible but then
it would break the 64-bit build because the 64-bit version of the library would
be absent. It would be a problem if Mono+LLVM actually used xml2, but since it
doesn't we can just disable the feature completely.