Makefile.inc1: bump elf tool chain version check after PR 215350 fix

This causes builds to use the in-tree libdwarf with the bug fix instead
of the host library.

Reviewed by:	bdrewery, emaste, markj
Sponsored by:	Dell EMC Isilon
Differential Revision:	https://reviews.freebsd.org/D12239

Makefile.inc1

@@ -1786,7 +1786,8 @@ update: .PHONY
 
 # ELF Tool Chain libraries are needed for ELF tools and dtrace tools.
 # r296685 fix cross-endian objcopy
-.if ${BOOTSTRAPPING} < 1100102
+# r310724 fixed PR 215350, a crash in libdwarf with objects built by GCC 6.2.
+.if ${BOOTSTRAPPING} < 1200020
 _elftoolchain_libs= lib/libelf lib/libdwarf
 .endif