Improve System.Native build experience for new platforms

[am11]

1. From top-level script, we can pass (only*) one extra argument to build-native.sh via BuildNativeClang as MSBuild property; /p:BuildNativeClang. This PR renames it to BuildNativeCompiler so it doesn't look confusing when selecting GCC compiler: corefx/build.sh /p:BuildNativeCompiler=-gcc.
2. When libssl devel package is not installed on the system, the default CMake's "library not found" message is unhelpful. This patch aligns the error handling of NOT OPENSSL_FOUND with those of KRB5 and CURL etc., so user knows which missing package to install.

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* Currently there is no way to pass more than one arguments via BuildNativeCompiler, as they get stripped somewhere in eng directory in corefx/build.sh -> ... -> corefx/src/Native/build-native.sh call chain. For example, we cannot do:

./build.sh /p:BuildNativeCompiler="cmakeargs -DSOME_CLR_PROPERTY=SomeValue cmakeargs -DCMAKE_C_FLAGS=-march=native" release cross arm64

because a pass-through script strips away the quotes.

[am11]

/cc @janvorli, @safern, @ViktorHofer

[ViktorHofer]

Thanks!

[am11]

Can this be merged? :)

[BruceForstall]

@am11 @ViktorHofer @stephentoub Note that the renaming of BuildNativeClang broke our corefx-in-coreclr-CI testing, which uses this argument (for Linux arm32, where we must specify the clang version).

If possible, please remember to notify us for any future change to this to avoid breaking us again.

[am11]

Sorry about that @BruceForstall, I was only grep'ing the CoreFX repo. Maybe we can try sharing/centralizing them via arcade (if possible). :)

[safern]

@BruceForstall sorry about that. Sure thing next time we will let you know next time.