rpm: stash files from BUILDROOT into BUILD after %build stage

When building RPM packages, stash files from BUILDROOT into BUILD
directory at the end of %build. Restore them back into BUILDROOT at the
beginning of the %install step. If this is not done, the script
generated by rpm for %install first removes all files in BUILDROOT,
losing all compiled files.
rpm: apply fix-embedded-paths to files in /usr/lib and /lib

Apply fix-embedded-paths to files in $RPM_BUILD_ROOT/usr/lib and
$RPM_BUILD_ROOT/lib, in addition to those found in
$RPM_BUILD_ROOT/opt/crosstool. If this is not done, check-buildroot will
flag these files as those containing the value of $RPM_BUILD_ROOT
inside.