devel/llvm10: fix hand-rolled AT_EXECPATH search for older FreeBSD

This was committed in base r360875 and upstream 21e5e1724, which was after
LLVM10 branched and has not been backported.

This failure would only have been noticed if AT_EXECPATH was missing, and
qemu-user-static does not currently provide AT_EXECPATH. As such, any
execution of llvm10 binaries under qemu-user-static immediately segfaulted.

Reported by: Daniel Tihanyi <daniel.tihanyi@tetragir.com>, jbeich
Approved by: brooks (maintainer)
MFH: 2020Q4