[llvm] Add support for the .base64 directive (fixes #165499) (PR #165549)

[Fangrui Song via llvm-commits]

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@@ -0,0 +1,28 @@
+# RUN: not llvm-mc -triple i386-unknown-unknown %s | FileCheck %s
----------------
MaskRay wrote:

Suggest: ensure there is a RUN line that exits with 0
```
# RUN: llvm-mc -triple=x86_64 %s | FileCheck %s
# RUN: not llvm-mc -triple=x86_64 -defsym=ERR=1 %s  -o /dev/null %s 2>&1 | ...
```

https://github.com/llvm/llvm-project/pull/165549