Fixed build errors with cross-clang compiler

[danielinux]

zd21232

[Copilot]

Pull request overview

Adjusts compiler- and toolchain-specific flags/attributes so the project builds cleanly with a cross-clang toolchain while preserving GCC behavior where needed.

Changes:

• Gate GCC-only pragmas/flags to avoid Clang incompatibilities.
• Adjust ARM build flags to omit -mthumb-interwork except under GCC.
• Tweak RAMFUNCTION attribute set for Clang vs GCC on ARM.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/update_flash.c	Avoids GCC-only #pragma GCC ... usage when compiling with Clang.
options.mk	Applies -Wstack-usage only when actually using GCC.
include/wolfboot/wolfboot.h	Switches RAMFUNCTION attributes for Clang vs GCC on ARM.
arch.mk	Removes -mthumb-interwork for non-GCC toolchains to prevent unsupported-flag errors.
Makefile	Drops -nostartfiles from default “headless GCC” CFLAGS.

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[dgarske]

@danielinux please resolve merge conflicts. I have not heard results from customer, but this looks good to merge. I'd also like to see a CI job attempt to build with CLANG.

[dgarske]

Sorry meant to request change.