i386/boot2: -fno-asynchronous-unwind-tables for gcc

The amd64 build of boot2 was failing with gcc 6.3.0 due to being more
than 1 kB too large. It was apparently generating a .eh_frame section
which was not being removed by objcopy -S. The .eh_frame section seems
to be mandatory per the amd64 ABI, but boot2 is compiled for i386 (uses
-m32), and therefore should be optional in this context. Suppress
generation of .eh_frame with the -fno-asynchronous-unwind-tables flag to
gcc. This saves 1348 bytes (the limit is 7680 bytes).

Reviewed by: dim, imp
Approved by: markj (mentor)
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D11928