[mkroot] Issue with mcm-buildall.sh

[Riccardo Paolo Bestetti]

I'm invoking mcm-buildall.sh to build a musl compiler for the x86_64
architecture:

(wd: $MCM)
$ ../mkroot/mcm-buildall.sh "x86_64::"

This:

1) afaics successfully builds the i686 bootstrap compiler in
   $MCM/host-i686-linux-musl
2) fails to build x86_64-linux-musl-cross (see below)
3) subsequently tries to build x86_64-linux-musl-native with the
   expected result of not finding a CC with x86_64-linux-musl-cross-
   prefix

Find the log for the second and third steps:
https://paste.sr.ht/~vmsh0/1f6d01d415eb66fe2d66c3a24cc6ede932daf574

In particular, the second step fails with:
make[2]: Leaving directory '/home/random/kernel-bug-repro/mcm/linux-headers-4.19.88'
find obj_kernel_headers/staged/include '(' -name .install -o -name ..install.cmd ')' -exec rm {} +
touch obj_kernel_headers/.lc_built
make[1]: Leaving directory '/home/random/kernel-bug-repro/mcm/build/local/x86_64-linux-musl'
make: *** [Makefile:185: install] Error 2
+ exit 1

Which, not being very proficient with make, I can't really make sense of. 

Riccardo