[llvm-nm] Add --portability as alias for --format=posix

[jhenderson]

Summary

--portability is a GNU nm alias for --format=posix. To ensure command-line compatibility, we need to add it to llvm-nm.

[mstorsjo]

LGTM

llvm/trunk/test/tools/llvm-nm/X86/posix-aliases.test

+# RUN: llvm-nm -P %p/Inputs/hello.obj.elf-x86_64 > %t1
+# RUN: llvm-nm -format=posix %p/Inputs/hello.obj.elf-x86_64 > %t2
+# RUN: llvm-nm -portability %p/Inputs/hello.obj.elf-x86_64 > %t3
+# RUN: diff %t1 %t2
+# RUN: diff %t1 %t3

llvm/trunk/tools/llvm-nm/llvm-nm.cpp

cl::opt<bool> NoWeakSymbols("no-weak",
                             cl::desc("Show only non-weak symbols"));
 cl::alias NoWeakSymbols2("W", cl::desc("Alias for --no-weak"),
                          cl::aliasopt(NoWeakSymbols), cl::Grouping);
 
 cl::opt<bool> BSDFormat("B", cl::desc("Alias for --format=bsd"),
                         cl::Grouping);
 cl::opt<bool> POSIXFormat("P", cl::desc("Alias for --format=posix"),
                           cl::Grouping);
+cl::alias Portability("portability", cl::desc("Alias for --format=posix"),
+                      cl::aliasopt(POSIXFormat), cl::NotHidden);
 cl::opt<bool> DarwinFormat("m", cl::desc("Alias for --format=darwin"),
                            cl::Grouping);
 
 static cl::list<std::string>
     ArchFlags("arch", cl::desc("architecture(s) from a Mach-O file to dump"),
               cl::ZeroOrMore);
 bool ArchAll = false;