[WebAssembly] Add support for -mexec-model=reactor

This adds a -mexec-model= command-line flag. The default value is "command"
which is no change from the current behavior. The other option is "reactor"
which enables the WASI Reactor ABI:

https://github.com/WebAssembly/WASI/blob/master/design/application-abi.md

Differential Revision: https://reviews.llvm.org/D62922
diff --git a/clang/docs/ClangCommandLineReference.rst b/clang/docs/ClangCommandLineReference.rst
index 4fde603..1ebcab1 100644
--- a/clang/docs/ClangCommandLineReference.rst
+++ b/clang/docs/ClangCommandLineReference.rst
@@ -3051,6 +3051,12 @@
 
 .. option:: -munimplemented-simd128, -mno-unimplemented-simd128
 
+.. option:: -mexec-model=<arg>
+
+Select between "command" and "reactor" executable models. Commands have a main
+function which scopes the lifetime of the program. Reactors are activated and
+remain active until explicitly terminated.
+
 X86
 ---
 .. option:: -m3dnow, -mno-3dnow