cmd/6c, cmd/8c: cut stack frames by about half
The routine that adds an automatic to the stack was
adding ptrsize-1 to the size before rounding up.
That addition would only make sense to turn a round down
into a round up. Before a round up, it just wastes a word.
The effect was that a 6c function with one local and
one two-word function call used (8+8)+(16+8) = 40 bytes
instead of 8+16 = 24 bytes.
The wasted space mostly didn't matter, but one place where
it does matter is when trying to stay within the 128-byte
total frame constraint for #pragma textflag 7 functions.
This only affects the C compilers, not the Go compilers.
5c already had correct code, which is now copied to 6c and 8c.
R=ken2
CC=golang-dev
https://golang.org/cl/
7303099