Make gen-XX-const scripts work with llvm-as

Make gen-XX-const scripts work with llvm-as

The gen-as-const and gen-py-const scripts are used to generate integer constant
definitions from a list of constant C-expressions. This is achieved by
generating a C program with inline `asm` statements, that depend on
these constant expressions. During compilation, the constant expressions
are evaluated, and included in the inline asm. The build process
generates only the assembly, and then used `sed` to extract the values
from the assembly text.

This is clever. It allows the build process to extract the value of C
statements built under the target architecture. The implementation is a
bit fragile, but it is not immediately obvious to me how it could be
improved.

This change slightly modifies `gen-as-const` and `gen-py-const` to emit
valid assembly directives instead of invalid directives that were
previously emitted. Since the values are extracted via string parsing,
this has no effect on the values extracted. This is needed because the
LLVM assembler validates all statements before emitting them, whereas it
appears GCC will literally emit any `asm` directives without validation
or recognition.