Consistently access function parameters using the FP pseudo-register
instead of SP (e.g., x+0(FP) instead of x+4(SP) or x+8(SP), depending
on register size). Two reasons: 1) doc/asm says the SP pseudo-register
should use negative offsets in the range [-framesize, 0), and 2)
cmd/vet only validates parameter offsets when indexed from the FP
pseudo-register.

No binary changes to the compiled object files for any of the affected
package/OS/arch combinations.

Change-Id: I0efc6079bc7519fcea588c114ec6a39b245d68b0
Reviewed-on: https://go-review.googlesource.com/30085Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>