boost: build libquadmath only on some archs

boost: build libquadmath only on some archs

libquadmath is only packaged for x86 and powerpc targets in OpenWrt.
Declaring it as an unconditional runtime dependency of boost-charconv
causes apk installation failure on all other architectures (e.g.
i386, arm, mips) with "libquadmath (no such package)".

Guard the dependency with an ARCH filter so it is only pulled in on
platforms where the package actually exists.

Signed-off-by: Alexandru Ardelean <redacted>
boost: add x86 to libquadmath arch conditional in boost-charconv

libquadmath is available on x86 (i386) in addition to x86_64 and
powerpc variants. Include it in the filter so boost-charconv gets
the correct dependency on 32-bit x86 targets.

Signed-off-by: Alexandru Ardelean <redacted>