[utils] Correctly convert MONO_DL_LOCAL to RTLD_LOCAL (mono/mono#18121)

[utils] Correctly convert MONO_DL_LOCAL to RTLD_LOCAL (mono/mono#18121)

On Linux, neither flag being specified will result in a default of RTLD_LOCAL. Unfortunately, on MacOS it's the opposite - the default is RTLD_GLOBAL. This means currently there is no way to set RTLD_LOCAL on Macs, so despite the potential for this to break someone it directly contradicts our documentation and needs to be changed.

Commit migrated from https://github.com/mono/mono/commit/0e05bda2356e453e6ab5ae8745c86bddf82b9ce6