Allow building without linking to libc

[alexrp]

Either if NoStdLib is set (but see dotnet/sdk#29110) or DisableImplicitFrameworkReferences.

[alexrp]

Putting this off for now because there's some nuance to be considered here. For example, would the user still expect to link to compiler-rt in this case?

[alexrp]

For example, would the user still expect to link to compiler-rt in this case?

Probably yes. This should just affect libc, i.e. add -nostdlib for C/C++ and omit -lc for Zig.