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

[rlibby]

Summary

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).

Test Plan

amd64/gcc buildworld

[dim]

LGTM.

[imp]

Looks reasonable to me. This switch is supported in gcc 4.2 and newer so we don't need anything conditional.

[markj]

Approved