internal/abi, internal/buildcfg: enable regabi on riscv64 by default

internal/abi, internal/buildcfg: enable regabi on riscv64 by default

This CL we turn regabi on by default.
Later CLs will clean up fallback code.

name                   old time/op    new time/op    delta
BinaryTree17              35.2s ± 2%     31.0s ± 1%  -11.87%  (p=0.008 n=5+5)
Fannkuch11                10.8s ± 1%     11.0s ± 1%   +1.73%  (p=0.008 n=5+5)
FmtFprintfEmpty           379ns ± 2%     389ns ± 4%     ~     (p=0.095 n=5+5)
FmtFprintfString          746ns ± 5%     693ns ± 2%   -7.10%  (p=0.008 n=5+5)
FmtFprintfInt             769ns ± 3%     744ns ± 2%   -3.23%  (p=0.032 n=5+5)
FmtFprintfIntInt         1.14µs ± 3%    1.09µs ± 4%   -4.51%  (p=0.032 n=5+5)
FmtFprintfPrefixedInt    5.04µs ± 0%    4.53µs ± 1%  -10.14%  (p=0.008 n=5+5)
FmtFprintfFloat          1.82µs ± 4%    1.67µs ± 4%   -8.12%  (p=0.008 n=5+5)
FmtManyArgs              3.92µs ± 3%    3.67µs ± 2%   -6.20%  (p=0.008 n=5+5)
GobDecode                 137ms ± 1%     132ms ± 1%   -4.13%  (p=0.008 n=5+5)
GobEncode                93.9ms ± 1%    89.3ms ± 2%   -4.84%  (p=0.008 n=5+5)
Gzip                      2.54s ± 1%     2.59s ± 1%   +1.95%  (p=0.008 n=5+5)
Gunzip                    1.39s ± 1%     1.35s ± 0%   -3.07%  (p=0.008 n=5+5)
HTTPClientServer          416µs ± 1%     397µs ± 0%   -4.39%  (p=0.016 n=5+4)
JSONEncode                202ms ± 1%     196ms ± 2%   -2.76%  (p=0.008 n=5+5)
JSONDecode                478ms ± 2%     435ms ± 4%   -8.97%  (p=0.008 n=5+5)
Mandelbrot200            44.1ms ± 0%    44.1ms ± 0%     ~     (p=0.151 n=5+5)
GoParse                  33.7ms ± 1%    31.5ms ± 1%   -6.55%  (p=0.008 n=5+5)
RegexpMatchEasy0_32       624ns ± 6%     548ns ± 6%  -12.08%  (p=0.008 n=5+5)
RegexpMatchEasy0_1K      4.49µs ± 1%    3.59µs ± 1%  -20.21%  (p=0.008 n=5+5)
RegexpMatchEasy1_32       654ns ± 4%     556ns ± 2%  -15.01%  (p=0.008 n=5+5)
RegexpMatchEasy1_1K      6.21µs ± 2%    4.96µs ± 3%  -20.22%  (p=0.008 n=5+5)
RegexpMatchMedium_32     9.78µs ± 5%    8.03µs ± 4%  -17.96%  (p=0.008 n=5+5)
RegexpMatchMedium_1K      280µs ± 4%     223µs ± 5%  -20.42%  (p=0.008 n=5+5)
RegexpMatchHard_32       17.0µs ± 3%    15.2µs ± 3%  -10.63%  (p=0.008 n=5+5)
RegexpMatchHard_1K        520µs ± 4%     485µs ± 6%   -6.76%  (p=0.008 n=5+5)
Revcomp                   35.2s ± 1%     35.2s ± 1%     ~     (p=0.690 n=5+5)
Template                  686ms ± 2%     610ms ± 3%  -11.09%  (p=0.008 n=5+5)
TimeParse                1.91µs ± 2%    1.78µs ± 1%   -6.86%  (p=0.008 n=5+5)
TimeFormat               3.04µs ± 2%    2.84µs ± 3%   -6.71%  (p=0.008 n=5+5)

name                   old speed      new speed      delta
GobDecode              5.59MB/s ± 1%  5.84MB/s ± 1%   +4.40%  (p=0.008 n=5+5)
GobEncode              8.17MB/s ± 1%  8.59MB/s ± 3%   +5.14%  (p=0.008 n=5+5)
Gzip                   7.65MB/s ± 1%  7.50MB/s ± 1%   -1.96%  (p=0.008 n=5+5)
Gunzip                 13.9MB/s ± 1%  14.4MB/s ± 0%   +3.16%  (p=0.008 n=5+5)
JSONEncode             9.63MB/s ± 1%  9.90MB/s ± 2%   +2.83%  (p=0.008 n=5+5)
JSONDecode             4.06MB/s ± 2%  4.46MB/s ± 4%   +9.90%  (p=0.008 n=5+5)
GoParse                1.72MB/s ± 1%  1.84MB/s ± 1%   +6.85%  (p=0.008 n=5+5)
RegexpMatchEasy0_32    51.4MB/s ± 6%  58.4MB/s ± 6%  +13.72%  (p=0.008 n=5+5)
RegexpMatchEasy0_1K     228MB/s ± 1%   286MB/s ± 1%  +25.32%  (p=0.008 n=5+5)
RegexpMatchEasy1_32    48.9MB/s ± 4%  57.5MB/s ± 2%  +17.63%  (p=0.008 n=5+5)
RegexpMatchEasy1_1K     165MB/s ± 2%   207MB/s ± 3%  +25.37%  (p=0.008 n=5+5)
RegexpMatchMedium_32   3.27MB/s ± 5%  3.99MB/s ± 5%  +21.87%  (p=0.008 n=5+5)
RegexpMatchMedium_1K   3.66MB/s ± 4%  4.60MB/s ± 5%  +25.61%  (p=0.008 n=5+5)
RegexpMatchHard_32     1.88MB/s ± 3%  2.10MB/s ± 3%  +12.03%  (p=0.008 n=5+5)
RegexpMatchHard_1K     1.97MB/s ± 4%  2.11MB/s ± 6%   +7.31%  (p=0.008 n=5+5)
Revcomp                7.23MB/s ± 1%  7.23MB/s ± 1%     ~     (p=0.810 n=5+5)
Template               2.83MB/s ± 2%  3.18MB/s ± 4%  +12.52%  (p=0.008 n=5+5)

Change-Id: Ie02aa5ebe60607dbb5007da0efae731c36e96399
Reviewed-on: https://go-review.googlesource.com/c/go/+/402374
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: mzh <mzh@golangcn.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>