Fix ucol_setMaxVariable detection for Gentoo Linux (dotnet/coreclr#5309)

Fix ucol_setMaxVariable detection for Gentoo Linux (dotnet/coreclr#5309)

The issue was the symbol is exported by the ICU lib. Including headers
was not enough. The linker requires the libraries to succeed.

With this fix, CoreCLR successfully builds on Gentoo Linux 100%.
Tested with LXC gentoo container on Ubuntu machine.

Steps to configure and build:
https://gist.github.com/jasonwilliams200OK/1a2e2c0e904ffa95faf6333fcd88d9b8

Fix dotnet/coreclr#5160

Commit migrated from https://github.com/dotnet/coreclr/commit/352df35c124104586ee4c977556cb50ce720af62