libc/armv7: remove default version for alloca

The alloca() interface has been a macro expanding to __builtin_alloca()
since 2003 (commit 79806b4cdce0, included in FreeBSD 5.2). Even before
that virtually every compiler version I was able to test replaced
alloca() with compiler generated code when targeting C.

Hide it to prevent future (mis)use.

Reviewed by: imp, kib
Differential Revision: https://reviews.freebsd.org/D51858