bootstrap: Reduced Binary Seed bootstrap for x86_64 too.

* gnu/packages/bootstrap.scm (%bootstrap-inputs): Replace %bootstrap-libc,
%bootstrap-gcc, and %bootstrap-binutils with Mes seeds for x86_64 too.
* gnu/packages/commencement.scm (tcc-boot0, tcc-boot, gcc-mesboot1,
gcc-mesboot1-wrapper, m4-mesboot, %bootstrap-inputs+toolchain,
libstdc++-boot0, gcc-boot0, bison-boot0): For x86_64-linux, use i686-linux
glibc-dynamic-linker and Mes bootstrap settings.
(gcc-boot0-intermediate-wrapped): New variable, extract from
static-bash-for-glibc.
(static-bash-for-glibc): Use it.
(gcc-final): Copy gmp&co phase from gcc-boot0, inherit other arguments
from gcc; avoid i686-linux impersonation from gcc-boot0.
* doc/guix.texi (Bootstrapping, Reduced Binary Seed Bootstrap): Note
x86_64-linux also as Reduced Binary Seed bootstrap.