libc/arm: use __builtin_trap() instead of abort() in aeabi_unwind stubs

This avoids a dependency on the abort symbol in libsys.

PR: 292539
Reviewed by: mmel
Approved by: markj (mentor)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D55255

(cherry picked from commit 1782bc9a0a8da2d6aca31b7790981e1980c9e4b9)