[gdb/testsuite] Skip gdb.rust/*.exp for target board unix/-m32

[gdb/testsuite] Skip gdb.rust/*.exp for target board unix/-m32

When running gdb.rust/*.exp with target board unix/-m32, we see:
...
Running src/gdb/testsuite/gdb.rust/union.exp ...
gdb compile failed, error: Unrecognized option: 'm'
Running src/gdb/testsuite/gdb.rust/modules.exp ...
gdb compile failed, error: Unrecognized option: 'm'
Running src/gdb/testsuite/gdb.rust/unsized.exp ...
gdb compile failed, error: Unrecognized option: 'm'
Running src/gdb/testsuite/gdb.rust/simple.exp ...
gdb compile failed, error: Unrecognized option: 'm'
Running src/gdb/testsuite/gdb.rust/watch.exp ...
gdb compile failed, error: Unrecognized option: 'm'
Running src/gdb/testsuite/gdb.rust/traits.exp ...
gdb compile failed, error: Unrecognized option: 'm'
Running src/gdb/testsuite/gdb.rust/expr.exp ...
Running src/gdb/testsuite/gdb.rust/rust-style.exp ...
gdb compile failed, error: Unrecognized option: 'm'
Running src/gdb/testsuite/gdb.rust/methods.exp ...
gdb compile failed, error: Unrecognized option: 'm'
Running src/gdb/testsuite/gdb.rust/generics.exp ...
gdb compile failed, error: Unrecognized option: 'm'
                === gdb Summary ===

nr of expected passes            95
nr of untested testcases         9
...

Fix this by testing for -m32 in the target board multilib_flags in
skip_rust_tests.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2021-01-20  Tom de Vries  <tdevries@suse.de>

	* lib/gdb.exp (skip_rust_tests): Skip if multilib_flags contains -m32.