[CMake] Get ELF SwiftRT building

[etcwilde]

These patches add the missing header search paths for building the swiftrt.o, as well as adding it to the default settings when building for BSD and Android to ensure that it isn't forgotten.

[etcwilde]

@swift-ci please smoke test

[etcwilde]

Linux failure:

******************** TEST 'lldb-shell :: SwiftREPL/BreakpointSimple.test' FAILED ********************
Exit Code: -9
Timeout: Reached timeout of 600 seconds

Command Output (stderr):
--
RUN: at line 3: /home/build-user/build/buildbot_linux/lldb-linux-x86_64/bin/lldb --no-lldbinit -S /home/build-user/build/buildbot_linux/lldb-linux-x86_64/test/Shell/lit-lldb-init-quiet --repl < /home/build-user/llvm-project/lldb/test/Shell/SwiftREPL/BreakpointSimple.test | /home/build-user/build/buildbot_linux/llvm-linux-x86_64/bin/FileCheck /home/build-user/llvm-project/lldb/test/Shell/SwiftREPL/BreakpointSimple.test
+ /home/build-user/build/buildbot_linux/lldb-linux-x86_64/bin/lldb --no-lldbinit -S /home/build-user/build/buildbot_linux/lldb-linux-x86_64/test/Shell/lit-lldb-init-quiet --repl
+ /home/build-user/build/buildbot_linux/llvm-linux-x86_64/bin/FileCheck /home/build-user/llvm-project/lldb/test/Shell/SwiftREPL/BreakpointSimple.test

--

[etcwilde]

@swift-ci please test Linux

[etcwilde]

@swift-ci please test macOS