build, qt: Use mkspecs/bitcoin-linux-g++ for all Linux hosts

Summary:
The hope is that this will make the guix compilation use the proper toolchains when compiling Qt and not pull in symbols from glibc 2.33

build: Adjust qt/guix_cross_lib_path.patch

doc: Remove no longer needed comments

Co-authored-by: fanquake <fanquake@gmail.com>

build: Add objcopy host tool

Qt's mkspec references the objcopy tool.

build, qt: Use mkspecs/bitcoin-linux-g++ for all Linux hosts

This is a backport of core#25838

Test Plan:
gitian builds and guix build (rebased on D14064)

The guix build still has one symbol-check failure after this patch (symbol pthread_sigmask from unsupported version GLIBC_2.32(15)), but this is progress as previously it had 5 failing symbols (stat, lstat64, fstat64 and stat64).

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14065