AstGen: allow non-string-literal inline assembly for stage1
AstGen: allow non-string-literal inline assembly for stage1

The end-game for inline assembly is that the syntax is more integrated
with zig, and it will not allow string concatenation for the assembler
code, for the same reasons that Zig does not have a preprocessor.

However, inline assembly in zig right now is lacking for a variety of
use cases (take a look at the open issues having to do with inline
assembly for example), and being able to use comptime expressions to
concatenate text is a workaround that real-world users are exploiting to
get by in the short term.

This commit keeps "assembly code must use string literal syntax" as a
compile error when using stage2, but allows it through when using
stage1.

I expect to revert this commit after making enough improvements to
inline assembly that our real world users' needs are satisfied.

andrewrk/zig,andrewrk/zig