gcc: fix runpath errors in cc1 binary

The runpath in the cc1 binary is:

Library runpath: [$ORIGIN/../../../recipe-sysroot-native/usr/lib:$ORIGIN/../../../recipe-sysroot-native/lib]

This does not match the actual location of the libraries, which would require:

Library runpath: [$ORIGIN/../../recipe-sysroot-native/usr/lib:$ORIGIN/../../recipe-sysroot-native/lib]

Prior to gcc 9.1 the recipe set B explicity with:

B = "${WORKDIR}/gcc-${PV}/build.${HOST_SYS}.${TARGET_SYS}"

and this build directory structure matches the runpath in cc1, so there is no issue.

This line was commented out in versions 9.1 through 11.3.  The upgrade to 12.1 once
again uncommented this line.

As a result the runpath is incorrect in version 9.1 through 11.3 and cc1 defaults
to using host libraries.

This patch restores setting B as done in master and versions prior to 9.1

Signed-off-by: Steve Sakoman <steve@sakoman.com>