nativeGen: Explicitly set flags of text sections on Windows

The binutils documentation (for COFF) claims,

> If no flags are specified, the default flags depend upon the section
> name. If the section name is not recognized, the default will be for the
> section to be loaded and writable.

We previously assumed that this would do the right thing for split
sections (e.g. a section named `.text$foo` would be correctly inferred
to be a text section). However, we have observed that this is not the
case (at least under the clang toolchain used on Windows): when
split-sections is enabled, text sections are treated by the assembler as
data (matching the "default" behavior specified by the documentation).

Avoid this by setting section flags explicitly. This should fix split
sections on Windows.

Fixes #22834.