runtime: change unused argument in fat write barriers from pointer to scalar

runtime: change unused argument in fat write barriers from pointer to scalar

The argument is unused, only present for alignment of the
following argument. The compiler today always passes a zero
but I'd rather not write anything there during the call sequence,
so mark it as a scalar so the garbage collector won't look at it.

As expected, no significant performance change.

name                                       old                     new          delta
BenchmarkBinaryTree17              17.9s × (0.99,1.00)     17.9s × (0.99,1.01)  ~
BenchmarkFannkuch11                4.35s × (1.00,1.00)     4.35s × (1.00,1.00)  ~
BenchmarkFmtFprintfEmpty           120ns × (0.94,1.05)     120ns × (0.93,1.06)  ~
BenchmarkFmtFprintfString          477ns × (1.00,1.00)     479ns × (0.99,1.00)  ~
BenchmarkFmtFprintfInt             450ns × (0.99,1.01)     452ns × (0.99,1.02)  ~
BenchmarkFmtFprintfIntInt          765ns × (0.99,1.01)     766ns × (0.99,1.01)  ~
BenchmarkFmtFprintfPrefixedInt     569ns × (0.99,1.01)     576ns × (0.98,1.01)  ~
BenchmarkFmtFprintfFloat           728ns × (1.00,1.00)     730ns × (1.00,1.01)  ~
BenchmarkFmtManyArgs              2.82µs × (0.99,1.01)    2.84µs × (0.99,1.00)  ~
BenchmarkGobDecode                39.1ms × (0.99,1.01)    39.3ms × (0.98,1.01)  ~
BenchmarkGobEncode                39.4ms × (0.99,1.01)    39.5ms × (0.99,1.01)  ~
BenchmarkGzip                      661ms × (0.99,1.01)     663ms × (1.00,1.01)  ~
BenchmarkGunzip                    143ms × (1.00,1.00)     143ms × (1.00,1.00)  ~
BenchmarkHTTPClientServer          133µs × (0.99,1.01)     132µs × (0.99,1.01)  ~
BenchmarkJSONEncode               57.3ms × (0.99,1.04)    57.4ms × (0.99,1.01)  ~
BenchmarkJSONDecode                139ms × (0.99,1.00)     139ms × (0.99,1.00)  ~
BenchmarkMandelbrot200            6.02ms × (1.00,1.00)    6.03ms × (1.00,1.00)  ~
BenchmarkGoParse                  9.72ms × (0.92,1.11)   10.31ms × (0.89,1.14)  ~
BenchmarkRegexpMatchEasy0_32       209ns × (1.00,1.01)     209ns × (1.00,1.00)  ~
BenchmarkRegexpMatchEasy0_1K       592ns × (0.99,1.00)     591ns × (0.99,1.00)  ~
BenchmarkRegexpMatchEasy1_32       183ns × (0.98,1.01)     184ns × (0.99,1.02)  ~
BenchmarkRegexpMatchEasy1_1K      1.01µs × (1.00,1.01)    1.01µs × (1.00,1.00)  ~
BenchmarkRegexpMatchMedium_32      330ns × (1.00,1.00)     330ns × (1.00,1.00)  ~
BenchmarkRegexpMatchMedium_1K     92.4µs × (1.00,1.00)    92.6µs × (1.00,1.00)  ~
BenchmarkRegexpMatchHard_32       4.77µs × (0.95,1.01)    4.80µs × (0.95,1.00)  ~
BenchmarkRegexpMatchHard_1K        136µs × (1.00,1.00)     136µs × (1.00,1.00)  ~
BenchmarkRevcomp                   906ms × (0.99,1.05)     900ms × (0.99,1.04)  ~
BenchmarkTemplate                  171ms × (0.99,1.01)     172ms × (1.00,1.00)  ~
BenchmarkTimeParse                 638ns × (1.00,1.00)     637ns × (1.00,1.00)  ~
BenchmarkTimeFormat                745ns × (0.99,1.02)     744ns × (1.00,1.01)  ~

Change-Id: I0aeac5dc7adfd75e2223e3aabfedc7818d339f9b
Reviewed-on: https://go-review.googlesource.com/9320
Reviewed-by: Austin Clements <austin@google.com>