c8a950d0d3b9 to 5.10, 5.4 and 4.19

Dear stable maintainers,

Please consider cherry-picking c8a950d0d3b9 ("tools: Factor HOSTCC, HOSTLD, HOSTAR definitions") to 5.10, 5.4 and 4.19. It fixes a problem where the host tools set by the user could be ignored for 'prepare-objtool', and is needed on x86 to enable the ORC unwinder, dynamic ftrace, etc. with LLVM=1 and a 'hermetic' toolchain environment.

On 5.10.10 it cherry-picks cleanly. On 5.4.92 and 4.19.170 it cherry-picks cleanly, besides 'tools/bpf/resolve_btfids/Makefile' which was introduced after 5.8-rc2. (This file can be ignored.)

Thanks!