* src/reader.c (reader_read_string): Support \xNN. Fixes Guix
bootstrap chain; i.e., compilation of gcc-2.95.3 using tcc-boot.
* tests/base.test ("reader: \\x08"): Test it.
* scaffold/tests/17-compare-char.c: Likewise, for mescc.
* build-aux/check-mescc.sh (tests): Run it.
