tests/tcg/xtensa: fix endianness issues in test_b
Use bbci.l/bbsi.l instead of bbci/bbsi, as they are assembly macros that
accept little-endian bit number and produce correct immediate for both
little and big endian configurations. Choose value loaded into register
for bbc/bbs opcodes based on configuration endianness.
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>