builder: use the SoC's existing shadow base with get_csr_header()

builder: use the SoC's existing shadow base with get_csr_header()

Both the SoC and get_csr_header() have independently set defaults
for the value of 'shadow_base'. If the SoC's value was modified,
ensure that get_csr_header() uses the modified value instead of
its own default.

Signed-off-by: Gabriel Somlo <somlo@cmu.edu>