[MC] [llvm-ml] Add support for the extrn keyword

[glandium]

Summary

It is the same as the already supported extern keyword.
https://learn.microsoft.com/en-us/cpp/assembler/masm/extrn?view=msvc-170

Fixes: https://github.com/llvm/llvm-project/issues/59712

[epastor]

Thanks for catching this alias!

[glandium]

@epastor could you land this for me?

[epastor]

In D140679#4018896, @glandium wrote:

@epastor could you land this for me?

Sure - in flight now. Thanks again for the contribution!

llvm/lib/MC/MCParser/MasmParser.cpp

void MasmParser::initializeDirectiveKindMap() {
   DirectiveKindMap["sqword"] = DK_SQWORD;
   DirectiveKindMap["real4"] = DK_REAL4;
   DirectiveKindMap["real8"] = DK_REAL8;
   DirectiveKindMap["real10"] = DK_REAL10;
   DirectiveKindMap["align"] = DK_ALIGN;
   DirectiveKindMap["even"] = DK_EVEN;
   DirectiveKindMap["org"] = DK_ORG;
   DirectiveKindMap["extern"] = DK_EXTERN;
+  DirectiveKindMap["extrn"] = DK_EXTERN;
   DirectiveKindMap["public"] = DK_PUBLIC;
   // DirectiveKindMap[".comm"] = DK_COMM;
   DirectiveKindMap["comment"] = DK_COMMENT;
   DirectiveKindMap["include"] = DK_INCLUDE;
   DirectiveKindMap["repeat"] = DK_REPEAT;
   DirectiveKindMap["rept"] = DK_REPEAT;
   DirectiveKindMap["while"] = DK_WHILE;
   DirectiveKindMap["for"] = DK_FOR;