cmd/internal/gc: accept comma-separated list of name=value for -d

cmd/internal/gc: accept comma-separated list of name=value for -d

This should obviously have no performance impact.
Listing numbers just as a sanity check for the benchmark
comparison program: it should (and does) find nothing
to report.

name                                       old                     new          delta
BenchmarkBinaryTree17              18.0s × (0.99,1.01)     17.9s × (0.99,1.00)  ~
BenchmarkFannkuch11                4.36s × (1.00,1.00)     4.35s × (1.00,1.00)  ~
BenchmarkFmtFprintfEmpty           120ns × (0.99,1.06)     120ns × (0.94,1.05)  ~
BenchmarkFmtFprintfString          480ns × (0.99,1.01)     477ns × (1.00,1.00)  ~
BenchmarkFmtFprintfInt             451ns × (0.99,1.01)     450ns × (0.99,1.01)  ~
BenchmarkFmtFprintfIntInt          766ns × (0.99,1.01)     765ns × (0.99,1.01)  ~
BenchmarkFmtFprintfPrefixedInt     569ns × (0.99,1.01)     569ns × (0.99,1.01)  ~
BenchmarkFmtFprintfFloat           728ns × (1.00,1.01)     728ns × (1.00,1.00)  ~
BenchmarkFmtManyArgs              2.81µs × (1.00,1.01)    2.82µs × (0.99,1.01)  ~
BenchmarkGobDecode                39.4ms × (0.99,1.01)    39.1ms × (0.99,1.01)  ~
BenchmarkGobEncode                39.4ms × (0.99,1.00)    39.4ms × (0.99,1.01)  ~
BenchmarkGzip                      660ms × (1.00,1.01)     661ms × (0.99,1.01)  ~
BenchmarkGunzip                    143ms × (1.00,1.00)     143ms × (1.00,1.00)  ~
BenchmarkHTTPClientServer          132µs × (0.99,1.01)     133µs × (0.99,1.01)  ~
BenchmarkJSONEncode               57.1ms × (0.99,1.01)    57.3ms × (0.99,1.04)  ~
BenchmarkJSONDecode                138ms × (1.00,1.01)     139ms × (0.99,1.00)  ~
BenchmarkMandelbrot200            6.02ms × (1.00,1.00)    6.02ms × (1.00,1.00)  ~
BenchmarkGoParse                  9.79ms × (0.92,1.07)    9.72ms × (0.92,1.11)  ~
BenchmarkRegexpMatchEasy0_32       210ns × (1.00,1.01)     209ns × (1.00,1.01)  ~
BenchmarkRegexpMatchEasy0_1K       593ns × (0.99,1.01)     592ns × (0.99,1.00)  ~
BenchmarkRegexpMatchEasy1_32       182ns × (0.99,1.01)     183ns × (0.98,1.01)  ~
BenchmarkRegexpMatchEasy1_1K      1.01µs × (1.00,1.01)    1.01µs × (1.00,1.01)  ~
BenchmarkRegexpMatchMedium_32      331ns × (1.00,1.00)     330ns × (1.00,1.00)  ~
BenchmarkRegexpMatchMedium_1K     92.6µs × (1.00,1.01)    92.4µs × (1.00,1.00)  ~
BenchmarkRegexpMatchHard_32       4.58µs × (0.99,1.05)    4.77µs × (0.95,1.01)  ~
BenchmarkRegexpMatchHard_1K        136µs × (1.00,1.01)     136µs × (1.00,1.00)  ~
BenchmarkRevcomp                   900ms × (0.99,1.06)     906ms × (0.99,1.05)  ~
BenchmarkTemplate                  171ms × (1.00,1.01)     171ms × (0.99,1.01)  ~
BenchmarkTimeParse                 637ns × (1.00,1.00)     638ns × (1.00,1.00)  ~
BenchmarkTimeFormat                742ns × (1.00,1.00)     745ns × (0.99,1.02)  ~

Change-Id: I59ec875715cb176bbffa709546370a6a7fc5a75d
Reviewed-on: https://go-review.googlesource.com/9309
Reviewed-by: Austin Clements <austin@google.com>