kernel: Disable errors for -Walloca-larger-than for GCC.

[jhb]

Summary

GCC complains about the use of alloca() with variable sizes (for XSAVE
state len) in sendsig() for i386. Modern XSAVE state is probably
getting a bit large for the i386 kstack, but downgrade the error to a
warning.

[kib]

I opted for alloca() initially because the only alternative there I see is to have one more fpu save area allocated per-thread, realistically. It might be a reasonable way now, esp, if combined with the move of the user fpu save area from kstack.

[emaste]

Is it maybe worth adding a comment in sendsig along the lines of the above note?