meson: add llvm-config to cross files

[kencu]

when building a non-native cross arch,
meson requires most binaries used during
the build to be specified in the cross
files.

these don't have to be full path-specified,
however, and simply setting this:

llvm-config = 'llvm-config'

will satisfy meson that it's OK to use the
llvm-config binary found in the PATH.

closes: https://trac.macports.org/ticket/71852

Description

Type(s)

• bugfix
• enhancement
• security fix

Tested on

macOS 15.2 24C101 arm64
Xcode 16.2 16C5032a

Verification

Have you

• followed our Commit Message Guidelines?
• squashed and minimized your commits?
• checked that there aren't other open pull requests for the same change?
• referenced existing tickets on Trac with full URL in commit message?
• checked your Portfile with port lint?
• tried existing tests with sudo port test?
• tried a full install with sudo port -vst install?
• tested basic functionality of all binary files?
• checked that the Portfile's most important variants haven't been broken?

[macportsbot]

Notifying maintainers:
@reneeotten for port py-meson.
@SoapZA for port py-meson.