arcv: Remove mtune=arc-v-rmx-100-series from t-arcv config list

[kolerov]

If GCC is built with --with-mtune=arc-v-rmx-100-series and mtune=arc-v-rmx-100-series is mentioned in MULTILIB_OPTIONS list of multilib options, then GCC cannot link an application properly in these cases:

1. When GCC is used without any multilib options with default options only.
2. When a multilib configuration does not involve mtune, e.g. march=rv64imafdc/mabi=lp64d/mcmodel=medany.

In both cases -mtune=arc-v-rmx-100-series is passed explicitly, but the corresponding configuration cannot be chosen since mtune=arc-v-rmx-100-series is not mentioned in t-arcv for them.

Since arc-v-rmx-100-series is a default -mtune value for ARC-V, then mtune=arc-v-rmx-100-series maybe omitted in t-arc and assumed to be a default -mtune value for all configurations without mtune=... field.

Thanks for taking the time to contribute to GCC! Please be advised that if you are
viewing this on github.com, that the mirror there is unofficial and unmonitored.
The GCC community does not use github.com for their contributions. Instead, we use
a mailing list ([email protected]) for code submissions, code reviews, and
bug reports. Please send patches there instead.

This PR fixes foss-for-synopsys-dwc-arc-processors/toolchain#656.

[artemiy-volkov]

I don't like this since we are now dependent on the user configuring with --with-tune=rmx100. I don't think rmx100 should get any special treatment here, this is an extremely fragile approach

[kolerov]

An alternative is to use rocket as the default mtune value. But in this case --mtune=arc-v-*-series must be always passed explicitly to GCC to link with ARC-V specific configurations (with mtune value in t-arcv).