build: add -fPIC for musl targets

[remorses]

Adds -fPIC flag for musl targets when building highway and simdutf C++ dependencies, matching the existing freebsd behavior.

What is PIC?

Position Independent Code (PIC) generates machine code using relative addresses instead of absolute ones, allowing the code to be loaded at any memory address. This is required when linking static libraries into shared libraries (.so files).

Why both freebsd and musl need it

Both freebsd and musl use strict relocation policies that reject non-PIC code in shared libraries. Without -fPIC, linking fails with errors like:

relocation R_X86_64_PC32 cannot be used against symbol '__cxa_begin_catch'

glibc is more permissive and handles these relocations at runtime, which is why linux-gnu targets work without this flag.

Context

This enables cross-compiling ghostty-vt to musl/Alpine Linux targets. Discovered while integrating ghostty-vt into opentui: anomalyco/opentui#440

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This PR was created with help from Claude Opus 4.5.

[Copilot]

Pull request overview

This PR adds the -fPIC compiler flag for musl ABI targets when building the highway and simdutf C++ dependencies, addressing linking issues when building static libraries for musl/Alpine Linux. This matches the existing behavior for FreeBSD targets, both of which enforce strict relocation policies.

Key Changes:

• Extended the FreeBSD-specific -fPIC condition to also include musl targets in both highway and simdutf build configurations

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
pkg/simdutf/build.zig	Updated conditional to add -fPIC flag for both FreeBSD and musl ABI targets
pkg/highway/build.zig	Updated conditional to add -fPIC flag for both FreeBSD and musl ABI targets

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[mitchellh]

Thanks! Ghostty (the GUI) in history has never worked with musl, but libghostty-vt has a good chance. I think we should add musl build targets to the CI for libghostty because this is a good thing to ensure...

[mitchellh]

I'll take over. Thanks!

[mitchellh]

Approved pending CI. I added a CI test case for musl and libghostty-vt.