[gdb/testsuite] Skip language auto in gdb.base/parse_number.exp

In test-case gdb.base/parse_number.exp, we skip architecture auto in the
$supported_archs loop, to prevent duplicate testing.

Likewise, skip language auto and its alias local in the $::all_languages
loop.  This reduces the number of tests from 17744 to 15572.

Tested on x86_64-linux, with a build with --enable-targets=all.