Drop linker script from DynamoRIOConfig.cmake and make as compiler-agnostic as possible

Xref https://groups.google.com/g/dynamorio-users/c/w-VRqWqym28/m/L-eHUbItAQAJ where today DynamoRIOConfig.cmake says:
```
  DynamoRIO's CMake configuration only supports the GNU linker on Linux
```
Even when using it for standalone and other modes rather than clients.

This issue covers several improvements:

+ Remove the linker script altogether and drop support for gnu ld older than 2009, which is when -Ttext-segment was added.

+ Audit the code for compiler-specific flags and make as many optional as possible. E.g., we have linker flags for gold and bfd ld for the base, but we could just not set a base if a different linker is used: the base is optional and not as important as it was back before private loaders and 64-bit reachability flexibility.

+ Remove the above message hopefully altogether, or limit to certain modes if there is some flag we need.