[gdb/testsuite] Support -fPIE/-fno-PIE/-pie/-no-pie in gdb_compile_rust

When running gdb.rust/*.exp test-cases with target board unix/-fPIE/-pie, I
run into:
...
builtin_spawn -ignore SIGHUP rustc --color never gdb.rust/watch.rs \
  -g -lm -fPIE -pie -o outputs/gdb.rust/watch/watch^M
error: Unrecognized option: 'f'^M
^M
compiler exited with status 1
...

The problem is that -fPIE and -fpie are gcc options, but for rust we use
rustc, which has different compilation options.

Fix this by translating the gcc options to rustc options in gdb_compile_rust,
similar to how that is done for ada in target_compile_ada_from_dir.

Likewise for unix/-fno-PIE/-no-pie.

Tested on x86_64-linux, with:
- native
- unix/-fPIE/-pie
- unix/-fno-PIE/-no-pie
specifically, on openSUSE Leap 15.2 both with package gcc-PIE:
- installed (making gcc default to PIE)
- uninstalled (making gcc default to non-PIE).
and rustc 1.52.1.

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