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This more closely restores what the old C runtime did.
(In C, g was an 'extern register' with the same effective
implementation as in this CL.)

On a late 2012 MacBookPro10,2, best of 5 old vs best of 5 new:

benchmark                          old ns/op      new ns/op      delta
BenchmarkBinaryTree17              4981312777     4463426605     -10.40%
BenchmarkFannkuch11                3046495712     3006819428     -1.30%
BenchmarkFmtFprintfEmpty           89.3           79.8           -10.64%
BenchmarkFmtFprintfString          284            262            -7.75%
BenchmarkFmtFprintfInt             282            262            -7.09%
BenchmarkFmtFprintfIntInt          480            448            -6.67%
BenchmarkFmtFprintfPrefixedInt     382            358            -6.28%
BenchmarkFmtFprintfFloat           529            486            -8.13%
BenchmarkFmtManyArgs               1849           1773           -4.11%
BenchmarkGobDecode                 12835963       117...