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

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.

Reviewed by: kib, emaste
Differential Revision: https://reviews.freebsd.org/D31934