libc/armv7: remove default version for alloca

The alloca() interface has been a macro expanding to __builtin_alloca()
since 2003 (commit 79806b4cdc, 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

lib/libc/arm/Symbol.map

@@ -5,7 +5,6 @@
  */
 FBSD_1.0 {
 	__mcount;
-	alloca;
 	brk;
 	sbrk;
 };

lib/libc/arm/gen/alloca.S

@@ -35,6 +35,7 @@
 /* like alloc, but automatic automatic free in return */
 
 #include <machine/asm.h>
+
 ENTRY(alloca)
 	add	r0, r0, #0x00000007	/* round up to next 8 byte alignment */
 	bic	r0, r0, #0x00000007
@@ -43,4 +44,6 @@ ENTRY(alloca)
 	RET
 END(alloca)
 
+	.symver alloca, alloca@FBSD_1.0
+
 	.section .note.GNU-stack,"",%progbits