There is always 128 bytes available below the stackguard. Allow functions
with medium-sized stack frames to use this, potentially allowing them to
avoid growing the stack.

This change makes all architectures use the same calculation as x86.

Change-Id: I2afb1a7c686ae5a933e50903b31ea4106e4cd0a0
Reviewed-on: https://go-review.googlesource.com/38734Reviewed-by: Cherry Zhang <cherryyz@google.com>